Extra-cardiovascular cardiac pacing system for delivering composite pacing pulses

ABSTRACT

An implantable medical device is configured to control a therapy module to couple a capacitor array comprising a plurality of capacitors to a plurality of extra-cardiovascular electrodes and control the therapy module to deliver a composite pacing pulse to a patient&#39;s heart via the plurality of extra-cardiovascular electrodes by sequentially discharging at least a portion of the plurality capacitors to produce a series of at least two individual pulses that define the composite pacing pulse.

REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/368,197, (U.S. Patent Publication No. 2017/0157399) filed Dec. 2,2016, which claims the benefit of the filing date of U.S. ProvisionalApplication No. 62/262,412 filed on Dec. 3, 2015, the content of both ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a system, device and method for delivering cardiac pacingpulses using extra-cardiovascular electrodes.

BACKGROUND

A variety of implantable medical devices (IMDs) for delivering atherapy, monitoring a physiological condition of a patient or acombination thereof have been clinically implanted or proposed forclinical implantation in patients. Some IMDs may employ one or moreelongated electrical leads carrying stimulation electrodes, senseelectrodes, and/or other sensors. IMDs may deliver therapy to or monitorconditions of a variety of organs, nerves, muscle or tissue, such as theheart, brain, stomach, spinal cord, pelvic floor, or the like.Implantable medical leads may be configured to position electrodes orother sensors at desired locations for delivery of electricalstimulation or sensing of physiological conditions. For example,electrodes or sensors may be carried along a distal portion of a leadthat is extended subcutaneously, transvenously, or submuscularly. Aproximal portion of the lead may be coupled to an implantable medicaldevice housing, which contains circuitry such as signal generationcircuitry and/or sensing circuitry.

Some IMDs, such as cardiac pacemakers or implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to theheart of the patient via electrodes carried by one or more implantableleads and/or the housing of the pacemaker or ICD.

The leads may be transvenous, e.g., advanced into the heart through oneor more veins to position endocardial electrodes in intimate contactwith the heart tissue. Other leads may be non-transvenous leadsimplanted outside the heart, e.g., implanted epicardially,pericardially, or subcutaneously. The electrodes are used to deliverelectrical stimulation pulses to the heart to address abnormal cardiacrhythms.

IMDs capable of delivering electrical stimulation for treating abnormalcardiac rhythms typically sense signals representative of intrinsicdepolarizations of the heart and analyze the sensed signals to identifythe abnormal rhythms. Upon detection of an abnormal rhythm, the devicemay deliver an appropriate electrical stimulation therapy to restore amore normal rhythm. For example, a pacemaker or ICD may deliver lowvoltage pacing pulses to the heart upon detecting bradycardia ortachycardia using endocardial or epicardial electrodes. An ICD maydeliver high voltage cardioversion or defibrillation shocks to the heartupon detecting fast ventricular tachycardia or fibrillation usingelectrodes carried by transvenous leads or non-transvenous leads. Thetype of therapy delivered and its effectiveness in restoring a normalrhythm depends at least in part on the type of electrodes used todeliver the electrical stimulation and their location relative to hearttissue.

SUMMARY

In general, the disclosure is directed to techniques for deliveringextra-cardiovascular pacing pulses by an implantable medical device. Apacemaker or ICD operating according to the techniques disclosed hereindelivers a series of fused low voltage electrical pulses usingextra-cardiovascular electrodes to produce a composite cardiac pacingpulse defined by the fused low voltage pulses. A composite pacing pulsedelivered using extra-cardiovascular electrodes may capture the heartwhen the cumulative pulse energy of the individual pulses exceeds acapture threshold of the heart, even when the individual pulse energiesare less than the capture threshold.

In one example, the disclosure provides an implantable medical devicecomprising a pacing control module and a therapy module. The therapymodule has a capacitor array comprising multiple capacitors and isconfigured to couple the capacitor array to extra-cardiovascularelectrodes to deliver a composite pacing pulse to a patient's heart. Thepacing control module is coupled to the therapy module and configured tocontrol the therapy module to sequentially discharge at least a portionof the capacitors to produce a series of at least two individual pulsesthat define the composite pacing pulse.

In another example, the disclosure provides a method performed by animplantable medical device. The method includes controlling a therapymodule to couple a capacitor array comprising multiple capacitors toextra-cardiovascular electrodes and controlling the therapy module by apacing control module to deliver a composite pacing pulse to a patient'sheart via the extra-cardiovascular electrodes by sequentiallydischarging at least a portion of the capacitors to produce a series ofat least two individual pulses that define the composite pacing pulse.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control module of an implantable medical device,cause the implantable medical device to control a therapy module tocouple a capacitor array comprising multiple capacitors toextra-cardiovascular electrodes and control the therapy module todeliver a composite pacing pulse to a patient's heart via theextra-cardiovascular electrodes by sequentially discharging at least aportion of the capacitors to produce a series of at least two individualpulses that define the composite pacing pulse.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are a conceptual diagrams of a patient implanted with an IMDsystem including a subcutaneously implanted IMD coupled to anextra-cardiovascular sensing, pacing and cardioversion/defibrillationlead for delivering extra-cardiovascular pacing pulses.

FIG. 1C is a schematic diagram of an alternative implant location of theextra-cardiovascular sensing, pacing and cardioversion/defibrillationlead of the IMD system of FIG. 1A.

FIG. 2A is a conceptual diagram illustrating a distal portion of anotherexample of the implantable electrical lead of FIG. 1A, having analternative electrode arrangement.

FIG. 2B is a conceptual diagram illustrating a distal portion of anotherexample of the extra-cardiovascular lead of FIG. 1A having an electrodearrangement similar to that of FIG. 2A but with a differently shapedlead body along a distal portion.

FIG. 3 is a schematic diagram of the IMD of FIG. 1A according to oneexample.

FIG. 4A is a depiction of an example of a composite pacing pulse thatmay be generated and delivered by the IMD of FIG. 1A to pace a patient'sheart using extra-cardiovascular electrodes.

FIG. 4B is a depiction of a composite pacing pulse according to anotherexample.

FIG. 5 is a schematic diagram of a pacing control module and a lowvoltage therapy module according to one example.

FIG. 6 is a schematic diagram of the pacing control module of FIG. 5according to one example.

FIG. 7 is a schematic diagram of a capacitor selection and controlmodule according to one example.

FIG. 8 is a flow chart of a method for delivering extra-cardiovascularpacing pulses according to one example.

FIG. 9 is a flow chart of a method that may be performed for selecting acapacitor sequence for delivering a composite pacing pulse.

FIG. 10 is a flow chart of a method for delivering a composite pacingpulse according to another example.

FIG. 11 is a conceptual diagram of an IMD coupled to transvenous leads.

FIG. 12A is a conceptual diagram of the IMD of FIG. 11 and a proximalportion of an extra-cardiovascular lead.

FIG. 12B is a conceptual diagram of the IMD and proximal portions of thetransvenous leads shown in FIG. 11.

FIG. 13 is a conceptual diagram of low voltage therapy module accordingto another example.

FIG. 14 is a conceptual diagram of an example of a composite pacingpulse that may be delivered by the low voltage therapy module of FIG.13.

FIG. 15 is a flow chart of a method for programmably configuring the IMDof FIG. 11 to operate to deliver either multi-channel, multi-chambercardiac pacing in conjunction with transvenous leads or to deliversingle-channel cardiac pacing in conjunction with anextra-cardiovascular lead and extra-cardiovascular electrodes.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for delivering lowvoltage pacing pulses using extra-cardiovascular electrodes that are notdirectly contacting cardiac tissue. As used herein, the term“extra-cardiovascular” refers to a position outside the blood vessels,heart, and pericardium surrounding the heart of a patient. Implantableelectrodes carried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but not in intimatecontact with myocardial tissue.

Pacing pulses delivered by endocardial or epicardial electrodes aregenerally not painful to a patient. Pacing pulses delivered byextra-cardiovascular electrodes may cause extra-cardiac capture ofnerves and recruitment of skeletal muscle that may cause a noticeablesensation to the patient and, in some instances, pain or discomfort tothe patient depending on the voltage amplitude of the pacing pulses. Thepulse voltage amplitude required to capture the heart when pacing withextra-cardiovascular electrodes, such as subcutaneous or submuscularelectrodes, may exceed an acceptable comfort level for the patient for agiven pacing pulse width. A pacing pulse having a lower voltageamplitude when delivered by extra-cardiovascular electrodes may requirea relatively long pulse width in order to deliver sufficient energy tocapture the heart. The long pulse width may be beyond the capacity of atypical low voltage pacing capacitor due to the relatively fast decayrate of the pulse amplitude. Since a pacing pulse is delivered as thepacing capacitor is discharged across the pacing electrode vector, thepacing pulse amplitude may decay below an effective voltage amplitudebefore the required pacing pulse width expires resulting in a pacingpulse having insufficient energy to capture the heart.

As disclosed herein, an implantable, extra-cardiovascular medical devicesystem is configured to deliver multiple individual electrical pulses insuccession within a selected pacing pulse width to produce a composite,low voltage pacing pulse having an overall pulse width that is longenough to successfully pace the heart and a low enough pulse amplitudethat, if perceptible by the patient, is acceptable. The composite pacingpulse may be delivered using extra-cardiovascular pacing electrodes thatare not in direct contact with the myocardial tissue. The energy of eachindividual pulse is “fused” in time or cumulative in effect to produce atotal pulse energy within the composite pulse width that is adequate tocause depolarization of myocardial tissue even when each individualpulse if delivered alone is inadequate to cause capture of themyocardial tissue.

The techniques disclosed herein may be implemented in any implantablepacemaker or ICD and particularly in a pacemaker or ICD coupled toextra-cardiovascular electrodes. The electrodes may be carried by amedical electrical lead extending from the pacemaker or ICD and/orcarried by the housing of the pacemaker or ICD. The techniques disclosedherein are not necessarily limited to implantable systems and may beimplemented in an external pacemaker or ICD using cutaneous surfaceelectrodes or transcutaneous electrodes.

FIGS. 1A and 1B are conceptual diagrams of a patient 12 implanted withan extra-cardiovascular IMD system 10 that includes a subcutaneouslyimplanted IMD 14 coupled to an extra-cardiovascular sensing, pacing andcardioversion/defibrillation (CV/DF) lead 16. FIG. 1A is a frontal viewof patient 12 and FIG. 1B is a transverse view of patient 12. In theillustrative embodiment of FIGS. 1A and 1B, IMD 14 is an ICD configuredfor delivering high-voltage cardioversion/defibrillation (CV/DF) shocksin addition to the low-voltage, extra-cardiovascular pacing pulsesdelivered using the techniques disclosed herein.

IMD 14 includes a housing 15 and connector assembly 17 for receivingextra-cardiovascular lead 16. IMD 14 acquires cardiac electrical signalsfrom heart 26 using electrodes carried by lead 16 and is configured todeliver cardiac pacing pulses to heart 26 using extra-cardiovascularelectrodes carried by lead 16. As will be described herein, IMD 14includes a pacing control module that controls an array of pacingcapacitors to deliver composite pacing pulses each comprising a seriesof fused low-voltage pulses. The composite pacing pulse has a pulseamplitude that may be set to a comfortable level to the patient, e.g.,less than 20 V, and a pulse width that is long enough to successfullycapture and pace the heart using extra-cardiovascular electrodes.

The cardiac electrical signals received by IMD 14 are used fordetermining the patient's heart rhythm and providing appropriate pacingtherapy as needed, such as bradycardia pacing, anti-tachycardia pacing(ATP), or pacing to treat asystole due to atrioventricular conductionblock or following a cardioversion or defibrillation shock, for example.When IMD 14 is embodied as an ICD, it is configured to detect shockablerhythms, e.g., non-sinus, fast ventricular tachycardia and ventricularfibrillation, and deliver CV/DF shock therapy via defibrillationelectrodes 24A and/or 24B carried by lead 16. In other examples, IMD 14may be configured as a pacemaker for delivering low voltage pacingtherapies without the capability of delivering high voltage CV/DF shocktherapy. In that case, lead 16 may not include the defibrillationelectrodes 24A and 24B.

Lead 16 includes a proximal end 27 that is connected to IMD 14 and adistal portion 25 that carries electrodes 24A, 24B, 28A, 28B and 30. Allor a portion of housing 15 of IMD 14 may be formed of a conductivematerial, such as titanium or titanium alloy, and coupled to internalIMD circuitry to function as an electrode, sometimes referred to as a“CAN electrode.”

Electrodes 24A and 24B are referred to as defibrillation electrodesbecause they may be used together or in combination with the conductivehousing 15 of IMD 14 for delivering high voltage CV/DF shocks.Electrodes 24A and 24B may be elongated coil electrodes and generallyhave a relatively high surface area for delivering high voltageelectrical stimulation pulses compared to low voltage pacing and sensingelectrodes 28A, 28B and 30. However, electrodes 24A and 24B may also beutilized to provide pacing functionality, sensing functionality or bothpacing and sensing functionality in addition to or instead of highvoltage stimulation therapy. In this sense, the use of the term“defibrillation electrode” herein should not be considered as limitingthe electrodes 24A and 24B to use in only high voltage CV/DF therapyapplications. As described herein, electrodes 24A and/or 24B may be usedin a pacing electrode vector for delivering compositeextra-cardiovascular pacing pulses.

In some cases, defibrillation electrodes 24A and 24B may together form adefibrillation electrode in that they are configured to be activatedconcurrently. Alternatively, defibrillation electrodes 24A and 24B mayform separate defibrillation electrodes in which case each of theelectrodes 24A and 24B may be activated independently. In someinstances, defibrillation electrodes 24A and 24B are coupled toelectrically isolated conductors, and IMD 14 may include switchingmechanisms to allow electrodes 24A and 24B to be utilized as a singledefibrillation electrode (e.g., activated concurrently to form a commoncathode or anode) or as separate defibrillation electrodes, (e.g.,activated individually, one as a cathode and one as an anode oractivated one at a time, one as an anode or cathode and the otherremaining inactive). In other examples, lead 16 may include a singledefibrillation electrode rather than two defibrillation electrodes asshown.

Electrodes 28A, 28B and 30 are relatively smaller surface areaelectrodes for delivering low voltage pacing pulses and for sensingcardiac electrical signals. Electrodes 28A, 28B and 30 are referred toas pace/sense electrodes because they are generally configured for usein low voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28A, 28B, and 30 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrodes 28A and 28Bare located between defibrillation electrodes 24A and 24B and electrode30 is located distal to defibrillation electrode 24A. Electrodes 28A and28B are illustrated as ring electrodes, and electrode 30 is illustratedas a hemispherical tip electrode in the example of FIG. 1A. However,electrodes 28A, 28B, and 30 may comprise any of a number of differenttypes of electrodes, including ring electrodes, short coil electrodes,paddle electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong lead body 18. Further, electrodes 28A, 28B, and 30 may be ofsimilar type, shape, size and material or may differ from each other.

An ECG signal may be acquired using a sensing vector including anycombination of electrodes 28A, 28B, 30 and housing 15. In some examples,a sensing vector may even include defibrillation electrodes 24A and/or24B. IMD 14 may include more than one sensing channel such that sensingelectrode vectors may be selected two at a time by IMD 14 for monitoringfor a shockable rhythm or determining a need for cardiac pacing.

Pacing pulses may be delivered using any combination of electrodes 24A,24B, 28A, 28B, 30 and housing 15. The pacing electrode vector selectedfor delivering pacing pulses may be selected based on pacing electrodevector impedance measurements and capture threshold testing. Forexample, a pacing vector may be selected from among electrodes 24A, 24B,28A, 28B, 30 and housing 15 that has the lowest impedance and/or thelowest composite pacing pulse width that captures the heart for aprogrammed pacing pulse voltage amplitude. The pacing pulse voltageamplitude may be programmed to be below a threshold for pain anddiscomfort, which may be based on individual patient testing and/orclinical data. In some examples, a pacing electrode vector is selectedbetween one of pace/sense electrodes 28A, 28B or 30 and one of thedefibrillation electrodes 24A or 24B. In other examples, a pacingelectrode vector is selected between two of the pace/sense electrodes28A, 28B and/or 30. Selection of a pacing electrode vector between twoelectrodes carried by the distal portion 25 of lead 16 may reduceskeletal muscle recruitment when extra-cardiovascular pacing pulses aredelivered compared to a pacing vector that includes IMD housing 15.

In some instances, electrodes 24A, 24B, 28A, 28B, and/or 30 of lead 16may be shaped, oriented, designed or otherwise configured to reduceextra-cardiac stimulation. For example, electrodes 24A, 24B and/orelectrodes 28A, 28B, and/or 30 of lead 16 may be shaped, oriented,designed, partially insulated or otherwise configured to focus, director point electrodes 24A, 24B and/or electrodes 28A, 28B, and/or 30toward heart 26. In this manner, electrical stimulation pulses deliveredvia lead 16 are directed toward heart 26 and not outward toward skeletalmuscle. For example, electrodes 24A, 24B and/or electrodes 28A, 28B,and/or 30 of lead 16 may be partially coated or masked with a polymer(e.g., polyurethane) or another coating material (e.g., tantalumpentoxide) on one side or in different regions so as to direct theelectrical energy toward heart 26 and not outward toward skeletalmuscle. In the case of a ring electrode, for example, the ring electrodemay be partially coated with the polymer or other material to form ahalf-ring electrode, quarter-ring electrode, or other partial-ringelectrode. When IMD 14 delivers pacing pulses via electrodes 24A, 24B,28A, 28B, and/or 30, recruitment of surrounding skeletal muscle by thepacing pulses, which can cause discomfort to the patient, may be reducedby shaping, orienting, or partially insulating electrodes to focus ordirect electrical energy toward heart 26.

In various examples, electrodes 24A, 24B, 28A, 28B and 30 may be carriedalong lead 16 at other locations than those shown and in differentarrangements relative to each other but are generally positioned toacquire cardiac electrical signals having acceptable cardiac signalstrength for sensing cardiac events, such as R-wave signals that occurupon depolarization of the ventricles, and for delivering low-voltagepacing pulses for successfully capturing the patient's heart 26. Whilethree pace/sense electrodes 28A, 28B and 30 are shown along lead 16,lead 16 may carry more or fewer pace/sense electrodes in other examples.Other arrangements of defibrillation and pace/sense electrodes carriedby an extra-cardiovascular lead that may be used for deliveringcomposite pacing pulses as described herein are generally disclosed inpending U.S. Pat. Publication No. 2015/0306375 (Marshall, et al.) andU.S. Pat. Publication No. 2015/0306410 (Marshall, et al.), both of whichare incorporated herein by reference in their entirety. In still otherexamples, lead 16 may carry a single pace/sense electrode to serve as apacing cathode (or anode) electrode with housing 15 or a defibrillationelectrode, e.g., defibrillation electrode 24A or 24B, serving as areturn anode (or cathode) electrode.

In other examples, dedicated pacing electrodes and separate, dedicatedsensing electrodes may be carried by lead 16 or another lead coupled toIMD 14. It is understood that one or more leads may be coupled to IMD 14for connecting at least one defibrillation electrode and at least onepacing and sensing electrode to IMD 14 for monitoring cardiac electricalsignals, delivering pacing pulses and delivering CV/DF shock therapywhen IMD 14 is configured as an ICD. Pacing therapies that may bedelivered by IMD 14 using any of electrodes 24A, 24B, 28A, 28B, 30 andhousing 15 may include, but are not limited to, bradycardia pacing, ATP,post-shock pacing for treating bradycardia or asystole after a CV/DFshock, pacing during asystole due to atrioventricular conduction block.Additionally or alternatively, composite pulses delivered according tothe techniques disclosed herein using any of electrodes 24A, 24B, 28A,28B, 30 and/or housing 15 may be entrainment pulses delivered prior to aT-shock for inducing a tachyarrhythmia or pulses included in a highfrequency burst of pulses (e.g., at 50 Hz) for inducing tachyarrhythmia,e.g., for the purposes of testing anti-tachyarrhythmia therapies in aclinical setting. The methods disclosed herein for delivering compositepacing pulses may be used in conjunction with the tachyarrhythmiainduction methods generally disclosed in U.S. Patent Application No.62/262,500 and corresponding U.S. Pat. No. 10,046,168, both incorporatedherein by reference in their entirety.

FIG. 1B is a transverse view of patient 12 showing the distal portion 25of lead 16 extending substernally, e.g., at least partially in oradjacent to the anterior mediastinum 36. Lead 16 is illustrated in FIG.1A and 1B as being implanted at least partially in a substernallocation, e.g., between the heart and ribcage 32 or sternum 22. In onesuch configuration, the proximal portion of lead 16 extendssubcutaneously from IMD 14 (which is implanted near a midaxillary lineon the left side of patient 12) toward sternum 22. At a location nearxiphoid process 20, lead 16 bends or turns superiorly and distal portion25 of lead 16, which carries electrodes 24A, 24B, 28A, 28B and 30,extends substernally, under or below the sternum 22 in the anteriormediastinum 36.

Anterior mediastinum 36 is bounded laterally by pleurae 39, posteriorlyby pericardium 38, and anteriorly by sternum 22. In some instances, theanterior wall of anterior mediastinum 36 may also be formed by thetransversus thoracis muscle and one or more costal cartilages. Anteriormediastinum 36 includes a quantity of loose connective tissue (such asareolar tissue), adipose tissue, some lymph vessels, lymph glands,substernal musculature, small side branches of the internal thoracicartery or vein, and the thymus gland. In one example, the distal portionof lead 16 extends along the posterior side of sternum 22 substantiallywithin the loose connective tissue and/or substernal musculature ofanterior mediastinum 36. Lead 16 may be at least partially implanted inother extra-cardiovascular, intrathoracic locations, e.g., along ribcage32 or along or adjacent to the perimeter of the pericardium or withinthe pleural cavity.

IMD 14 may also be implanted at other subcutaneous locations on patient12, such as further posterior on the torso toward the posterior axillaryline, further anterior on the torso toward the anterior axillary line,in a pectoral region, or at other locations of patient 12. In instancesin which IMD 14 is implanted pectorally, lead 16 may follow a differentpath, e.g., across the upper chest area and inferior along sternum 22.When the IMD 14 is implanted in the pectoral region, the system 10 mayinclude a second lead that extends along the left side of the patientand includes a defibrillation electrode and/or one or more pacingelectrodes positioned along the left side of the patient to function asan anode or cathode of a therapy delivery vector including anotherelectrode located anteriorly for delivering electrical stimulation toheart 26 positioned there between.

FIG. 1C is a schematic diagram of an alternative implant location oflead 16. In other examples, the distal portion of lead 16 may beimplanted at other extra-cardiovascular locations than the substernallocation shown in FIG. 1A. For instance, as shown in FIG. 1C, lead 16may be implanted subcutaneously or submuscularly, between the skin andthe ribcage 32 or between the skin and sternum 22. Lead 16 may extendsubcutaneously from IMD 14 toward xiphoid process 20 as shown in FIG.1A, but instead of extending substernally, along the posterior side ofsternum 22, lead 16 may bend or turn at a location near xiphoid process20 and extend subcutaneously or submuscularly superior, over sternum 22and/or ribcage 32. The distal portion 25 of lead 16 may be parallel tosternum 22 or laterally offset from sternum 22, to the left or theright. In other examples, the distal portion 25 of lead 16 may be angledlaterally away from sternum 22, either to the left or the right, suchthat the distal portion 25 extends non-parallel to sternum 22.

In another example, IMD 14 may be implanted subcutaneously outside theribcage 32 in an anterior medial location. Lead 16 may be tunneledsubcutaneously into a location adjacent to a portion of the latissimusdorsi muscle of patient 12, from a medial implant pocket of IMD 14laterally and posterially to the patient's back to a location oppositeheart 26 such that the heart 26 is generally disposed between the IMD 14and electrodes 24A, 24B, 28A, 28B and 30. The techniques disclosedherein for generating low voltage pacing pulses for pacing the heartusing extra-cardiovascular electrodes are not limited to a particularsubcutaneous, submuscular, supra-sternal, substernal or intra- orextra-thoracic location of the extra-cardiovascular electrodes.

Referring again to FIG. 1A, lead 16 includes an elongated lead body 18that carries the electrodes 24A, 24B, 28A, 28B and 30 and insulateselongated electrical conductors (not illustrated) that extend from arespective electrode 24A, 24B, 28A, 28B and 30 through the lead body 18to a proximal connector (not shown) that is coupled to connectorassembly 17 of IMD 14 at lead proximal end 27. Lead body 18 may beformed from a non-conductive material, such as silicone, polyurethane,fluoropolymers, or mixtures thereof or other appropriate materials, andis shaped to form one or more lumens within which the one or moreconductors extend. The conductors are electrically coupled to IMDcircuitry, such as a therapy delivery module and/or a sensing module,via connections in IMD connector assembly 17 that includes a connectorbore for receiving the proximal connector of lead 16 and associatedelectrical feedthroughs crossing IMD housing 15. The electricalconductors transmit electrical stimulation therapy from a therapydelivery module within IMD 14 to one or more of electrodes 24A, 24B,28A, 28B and/or 30 and transmit cardiac electrical signals from one ormore of electrodes 24A, 24B, 28A, 28B and/or 30 to the sensing modulewithin IMD 14.

Housing 15 forms a hermetic seal that protects internal electroniccomponents of IMD 14. As indicated above, housing 15 may function as a“CAN electrode” since the conductive housing or a portion thereof may beelectrically coupled to internal circuitry to be used as an indifferentor ground electrode during ECG sensing or during therapy delivery. Aswill be described in further detail herein, housing 15 may enclose oneor more processors, memory devices, transmitters, receivers, sensors,sensing circuitry, therapy circuitry and other appropriate components.

The example system 10 of FIG. 1A is illustrative in nature and shouldnot be considered limiting of the techniques described in thisdisclosure. The techniques disclosed herein may be implemented innumerous ICD or pacemakers and electrode configurations that includeextra-cardiovascular electrodes for delivering cardiac pacing pulses.The IMD system 10 is referred to as an extra-cardiovascular IMD systembecause lead 16 is a non-transvenous lead, positioned outside the bloodvessels, heart 26 and pericardium 38. The techniques disclosed hereinmay also be employed by a leadless device implanted substernally,intra-thoracically or extra-thoracically and having electrodes carriedby the housing, and/or in some cases by a conductor extending from thehousing. Another example of an IMD in which the presently disclosedtechniques may be implemented is generally disclosed in U.S. Pat. No.8,758,365 (Bonner, et al.), incorporated herein by reference in itsentirety.

An external device 40 is shown in telemetric communication with IMD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with IMD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenIMD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from IMD 14 and to programoperating parameters and algorithms in IMD 14 for controlling IMDfunctions. External device 40 may be used to program cardiac rhythmdetection parameters and therapy control parameters used by IMD 14. Datastored or acquired by IMD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from IMD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand held device.

FIG. 2A is a conceptual diagram illustrating a distal portion 25′ ofanother example of implantable electrical lead 16 having an alternativeelectrode arrangement. In this example, distal portion 25′ includes twopace/sense electrodes 28A and 28B and two defibrillation electrodes 24Aand 24B and respective conductors to provide the electrical stimulationand sensing functionality as described above in conjunction with FIG.1A. In this example, however, electrode 28B is proximal to proximaldefibrillation electrode 24B, and electrode 28A is distal to proximaldefibrillation electrode 24B such that pace/sense electrodes 28A and 28Bare separated by defibrillation electrode 24B. In a further example, inaddition to electrodes 28A and 28B, lead 16 may include a thirdpace/sense electrode located distal to defibrillation electrode 24A. IMD14 may deliver cardiac pacing pulses and/or sense electrical signalsusing any electrode vector that includes defibrillation electrodes 24Aand/or 24B (individually or collectively), and/or electrodes 28A and/or28B, and/or the housing 15 of IMD 14.

The spacing and location of pace/sense electrodes 28A and 28B may beselected to provide pacing vectors that enable efficient pacing of heart26. The lengths and spacing of electrodes 24A, 24B, 28A and 28B maycorrespond to any of the examples provided in the above-incorporatedreferences. For example, the distal portion 25′ of lead 16 from thedistal end to the proximal side of the most proximal electrode (e.g.,electrode 28B in the example of FIG. 2A) may be less than or equal to 15cm and may be less than or equal to 13 cm and or even less than or equalto 10 cm. It is contemplated that one or more pace/sense electrodes maybe distal to distal defibrillation electrode 24A, one or more pace/senseelectrodes may be between defibrillation electrodes 24A and 24B, and/orone or more pace/sense electrodes may be proximal to proximaldefibrillation electrode 24B. Having multiple electrodes at differentlocations along lead body 18 enables selection from among a variety ofinter-electrode spacings, which allows a pacing electrode pair (orcombination) to be selected having an inter-electrode spacing thatresults in the greatest pacing efficiency.

FIG. 2B is a conceptual diagram illustrating a distal portion 25″ofanother example of extra-cardiovascular lead 16 having an electrodearrangement similar to that of FIG. 2A but with a non-linear or curvingdistal portion 25″ of lead body 18′. Lead body 18′ may be pre-formed tohave a normally curving, bending, serpentine, undulating, or zig-zaggingshape along distal portion 25″. In this example, defibrillationelectrodes 24A′ and 24B′ are carried along pre-formed curving portionsof the lead body 18′. Pace/sense electrode 28A′ is carried in betweendefibrillation electrodes 24A′ and 24B′. Pace/sense electrode 28B′ iscarried proximal to the proximal defibrillation electrode 24B′.

In one example, lead body 18′ may be formed having a curving distalportion 25″ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “ε.” Defibrillation electrodes 24A′and 24B′ are each carried by the two respective C-shaped portions of thelead body distal portion 25″ and extend or curve in the same direction.In the example shown, pace/sense electrode 28A′ is proximal to theC-shaped portion carrying electrode 24A′, and pace/sense electrode 28B′is proximal to the C-shaped portion carrying electrode 24B′. Pace/senseelectrodes 24A′ and 24B′ are approximately aligned with a central axis31 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24A′ and 24B′ are laterallyoffset from electrodes 28A′ and 28B′. Defibrillation electrodes 24A′ and24B′ are located along respective C-shaped portions of the lead bodydistal portion 25″ that extend laterally in the same direction away fromcentral axis 31 and electrodes 28A′ and 28B′. Other examples ofextra-cardiovascular leads including one or more defibrillationelectrodes and one or more pacing and sensing electrodes carried by acurving, serpentine, undulating or zig-zagging distal portion of thelead body that may be implemented with the pacing techniques describedherein are generally disclosed in pending U.S. Pat. Publication No.2016/0158567 (Marshall, et al.), incorporated herein by reference in itsentirety.

FIG. 3 is a schematic diagram of IMD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asa can electrode in FIG. 5) includes software, firmware and hardware thatcooperatively monitor one or more ECG signals, determine when a pacingtherapy is necessary, and deliver prescribed pacing therapies as needed.When IMD 14 is configured as an ICD as illustrated herein, the software,firmware and hardware is also configured to determine when a CV/DF shockor cardiac pacing is necessary, and deliver prescribed CV/DF shocktherapies or pacing therapies. IMD 14 may be coupled to a lead, such aslead 16 shown in any of the examples of FIGS. 1A, 1B, 1C, 2A and 2B,carrying extra-cardiovascular electrodes 24A, 24B, 28A, and 28B and insome examples electrode 30 (not shown in FIG. 3), for delivering pacingtherapies, CV/DF shock therapies and sensing cardiac electrical signals.

IMD 14 includes a control module 80, memory 82, therapy delivery module84, electrical sensing module 86, telemetry module 88, and may includean impedance measurement module 90 and an optional sensor module 92. Apower source 98 provides power to the circuitry of IMD 14, includingeach of the modules 80, 82, 84, 86, 88, 90 and 92 as needed. Powersource 98 may include one or more energy storage devices, such as one ormore rechargeable or non-rechargeable batteries. Power source 98 iscoupled to low voltage (LV) and high voltage (HV) charging circuitsincluded in therapy delivery module 84 for charging LV and HVcapacitors, respectively, included in therapy delivery module 84 forgenerating therapeutic electrical stimulation pulses.

The functional blocks shown in FIG. 3 represent functionality includedin IMD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to IMD 14 herein. As used herein,the term “module” refers to an ASIC, an electronic circuit, a processor(shared, dedicated, or group) and memory that execute one or moresoftware or firmware programs, a combinational logic circuit, statemachine, or other suitable components that provide the describedfunctionality. The particular form of software, hardware and/or firmwareemployed to implement the functionality disclosed herein will bedetermined primarily by the particular system architecture employed inthe device and by the particular detection and therapy deliverymethodologies employed by the IMD. Depiction of different features asmodules is intended to highlight different functional aspects and doesnot necessarily imply that such modules must be realized by separatehardware or software components. Rather, functionality associated withone or more modules may be performed by separate hardware or softwarecomponents, or integrated within common hardware or software components.For example, pacing therapy control operations performed by controlmodule 80 may be implemented in a processor executing instructionsstored in memory 82. IMD 14 may include more or fewer modules than shownin FIG. 3. For example impedance measuring module 90 and sensor module92 may be optional and excluded in some instances. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern IMD system, given the disclosure herein, iswithin the abilities of one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control module 80 orother IMD modules to perform various functions attributed to IMD 14 orthose IMD modules. The non-transitory computer readable media storingthe instructions may include any of the media listed above.

Control module 80 communicates with therapy delivery module 84 andelectrical sensing module 86 for sensing cardiac electrical activity,detecting cardiac rhythms, and controlling delivery of cardiacelectrical stimulation therapies in response to sensed cardiac signals.Therapy delivery module 84 and electrical sensing module 86 areelectrically coupled to electrodes 24A, 24B, 28A, 28B carried by lead 16and the housing 15, which may function as a common or ground electrode.

Electrical sensing module 86 is selectively coupled to electrodes 28Aand 28B and housing 15 in order to monitor electrical activity of thepatient's heart. Electrical sensing module 86 may additionally beselectively coupled to electrodes 24A and 24B. Sensing module 86 isenabled to selectively monitor one or more sensing vectors selected fromthe available electrodes 24A, 24B, 28A and 28B and housing 15. Forexample, sensing module 86 may include switching circuitry for selectingwhich of electrodes 24A, 24B, 28A and 28B and housing 15 are coupled tosense amplifiers or other cardiac event detection circuitry included insensing module 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple sense amplifiers to selected electrodes.The cardiac event detection circuitry within electrical sensing module86 may include one or more sense amplifiers, filters, rectifiers,threshold detectors, comparators, analog-to-digital converters (ADCs),or other analog or digital components.

In some examples, electrical sensing module 86 includes multiple sensingchannels for acquiring cardiac electrical signals from multiple sensingvectors selected from electrodes 24A, 24B, 28A and 28B and housing 15.Each sensing channel may be configured to amplify, filter and rectifythe cardiac electrical signal received from selected electrodes coupledto the respective sensing channel to improve the signal quality forsensing cardiac events, e.g., R-waves and P-waves.

For example, each sensing channel in sensing module 86 may include aninput or pre-filter and amplifier for receiving a cardiac electricalsignal from a respective sensing vector, an analog-to-digital converter,a post-amplifier and filter, a rectifier to produce a digitized,rectified and amplified cardiac electrical signal that is passed to acardiac event detector included in sensing module 86 and/or to controlmodule 80. The cardiac event detector may include a sense amplifier,comparator or other circuitry for comparing the rectified cardiacelectrical signal to a cardiac event sensing threshold, such as anR-wave sensing threshold, which may be an auto-adjusting threshold.Sensing module 84 may produce a sensed cardiac event signal in responseto a sensing threshold crossing. Cardiac event sensing thresholds usedby each sensing channel may be automatically adjusted according tosensing control parameters, which may be stored in memory 82.

Sensed event signals produced by electrical sensing module 86 may beused by control module 80 for detecting a shockable rhythm and/or fordetecting a need for pacing. For example, control module 80 may respondto sensed event signals by setting pacing escape intervals forcontrolling the timing of pacing pulses delivered by therapy deliverymodule 84. In addition to the sensed cardiac event signals, electricalsensing module 86 may output a digitized ECG signal for use by controlmodule 80 in detecting/confirming tachycardia, e.g., via a morphology orwavelet analysis.

Therapy delivery module 84 includes an LV therapy module 85 fordelivering low voltage pacing pulses using an extra-cardiovascularpacing electrode vector selected from electrodes 24A, 24B, 28A and 28Band housing 15. The LV therapy module 85 includes an array of capacitorsthat are selectably controlled by control module 80 to provide a singlecomposite pacing pulse comprising a series of two or more fused pulsesdelivered individually by sequentially discharging capacitors of thecapacitor array within a composite pacing pulse width. Multiplecapacitors may be selected one at a time in sequence to deliver theindividual pulses included within a composite pacing pulse. In otherinstances, multiple capacitors may be selected two at a time to delivereach individual pulse of the composite pacing pulse. In variousexamples, two or more combinations of one or more capacitors areselected in timed sequence to deliver two or more sequentially fusedpulses which collectively define the composite pacing pulse. As usedherein, the term “fused pulses” refers to electrical pulses that aredelivered sequentially within a composite pacing pulse width to producea cumulative pulse energy sufficient to cause a pacing-evoked myocardialdepolarization to capture the heart. The pulse energy of each individualone of the fused pulses may be insufficient to capture the heart, butthe cumulative energy of the fused pulses delivered within the timeenvelope of the composite pacing pulse width is sufficient to cause asingle evoked response of the myocardium.

LV capacitors included in the LV therapy module 85 are charged to avoltage according to a programmed pacing pulse amplitude by a LVcharging circuit, which may include a state machine for charging the LVcapacitors to a multiple of a battery charge included in power source98, for example four times the battery charge. At an appropriate time,the LV therapy module 85 couples individual capacitors (or combinationsof individual capacitors) of the capacitor array to a pacing electrodevector in timed sequence. The capacitor combinations are sequentiallydischarged to deliver a composite pacing pulse defined by thesequentially delivered individual pulses. The individually deliveredpulses are fused in time such that the individual pulse energy iscumulative in producing a total pulse energy that is greater than thepacing capture threshold of the patient's heart, even though eachindividual pulse may have a pulse energy that is less than the pacingcapture threshold. In some examples, the leading edge of an individualpulse delivered by one capacitor (or one combination of capacitors)occurs at (e.g., within inherent electrical circuitry timinglimitations) or before the immediately preceding individual pulsereaches its terminating edge. In some cases, the individual pulses maybe separated by a non-zero time gap within the composite pacing pulsewidth, however the cumulative electrical energy of the individual pulseswithin the composite pulse width is sufficient to capture themyocardium.

As described below, LV therapy module 85 may be configured to sample thecomposite pacing pulse amplitude in real time over the composite pacingpulse width. The next capacitor (or combination of capacitors) in thecapacitor sequence may be coupled to the pacing electrode vector whenthe pulse amplitude of an individual pulse reaches a thresholdamplitude. In this way, the composite pacing pulse amplitude is notallowed to fall below a predetermined minimum amplitude for the entiretyof the composite pacing pulse width.

Impedance measurement module 90 may be electrically coupled to theavailable electrodes 24A, 24B, 28A and 28B and housing 15 for performingimpedance measurements of one or more candidate pacing electrodevectors. Control module 80 may control impedance measurement module 90to perform impedance measurements for use in selecting the pacingelectrode vector. For example, control module 80 may pass a signal toimpedance measurement module 90 to initiate an impedance measurement fora pacing electrode vector. Impedance measurement module 90 is configuredto apply a drive or excitation current across a pacing electrode vectorand determine the resulting voltage. The voltage signal may be useddirectly as the impedance measurement or impedance may be determinedfrom the applied current and the measured voltage. The impedancemeasurement may be passed to control module 80 for use in selecting apacing electrode vector for therapy delivery.

As described below, an impedance measurement may be used by controlmodule 80 for selecting the number of individual pulses and/or theseries of capacitors that will be discharged to produce the compositepacing pulse. The impedance of the selected pacing vector and thecapacitance of a given capacitor or capacitor combination in thecapacitor array will determine the decay rate of an individual pulse. Ifthe impedance is relatively low, the individual pulse has a relativelyfast decay rate. A fast decay of an individual pulse requires the nextpulse in a series of fused pulses to occur early, before the first pulsedecays below a minimum amplitude for a time period that would lead toloss of capture. The next capacitor (or capacitor combination) in theseries may be required to begin discharging relatively earlier if theimpedance is low and the decay rate is fast compared to when theimpedance is high and the pacing pulse decay rate is relatively slower.The individual pulse width, therefore, may be longer when the impedanceis relatively high, and the individual pulse width may be relativelyshorter when the impedance is relatively low.

To achieve a desired overall pulse width and sustain the composite pulseamplitude above a minimum amplitude for all or a vast majority of thecomposite pulse width, the individual pulse width may be relativelyshort, and the pulse number may be increased when pacing vectorimpedance is low. Fewer relatively longer individual pulses may bedelivered when the pacing vector impedance is relatively higher. Theindividual pulse width may be decreased and the individual pulse numbermay be increased in order to achieve a desired composite pulse widthwith a pulse amplitude that remains above a minimum amplitude thresholdfor each individual pulse width. The time interval from one leading edgeto the next leading edge of the individual pulses and the total numberof pulses will determine the overall composite pacing pulse width.

If the impedance is low, the decay time of the pulse may be lengthenedby using a higher capacitance for delivering each individual pulse. Theindividual pulse width and pulse number may be kept the same, but thedecay time is adjusted by selecting a capacitor or combination ofcapacitors having a higher capacitance value when pacing vectorimpedance is low. Accordingly, impedance measurements from impedancemeasurement module 90 may be used by control module 80 for determining arequired number of individual pulses, determining individual pulse widthand/or determining the capacitance used to produce each individual pulseof the composite pacing pulse in order to produce a composite pacingpulse having an amplitude profile for the duration of the pulse widththat successfully captures and paces the heart.

In embodiments in which IMD 14 provides high voltage therapy such ascardioversion/defibrillation shock pulses, therapy delivery module 84may additionally include high voltage (HV) therapy module 83 includingone or more high voltage output capacitors. HV therapy module 83 may beoptional and omitted when IMD 14 is provided for delivering pacingpulses without the capability of high voltage therapies. When include,IMD 14 may be configured to detection a shockable rhythm such asventricular fibrillation or fast ventricular tachycardia. In response todetecting a shockable rhythm, the HV capacitors are charged to aprogrammed voltage level by a HV charging circuit. The HV chargingcircuit may include a transformer and be a processor-controlled chargingcircuit that is controlled by control module 80. Control module 80applies a signal to trigger discharge of the HV capacitors upondetecting a feedback signal from therapy delivery module 84 that the HVcapacitors have reached the voltage required to deliver a programmedshock energy. In this way, control module 80 controls operation of thehigh voltage therapy module 83 to deliver CV/DF shocks usingdefibrillation electrode 24 and housing 15.

High energy CV/DF shocks are generally on the order of at least 5 Joulesand more commonly on the order of 20 Joules or higher. For the sake ofcomparison, the HV capacitor(s) of the HV therapy module 83, whenincluded, may be charged to an effective voltage greater than 100 V fordelivering a cardioversion/defibrillation shock. For example, two orthree HV capacitors may be provided in series having an effectivecapacitance of 148 microfarads in HV therapy module 83. These seriescapacitors may be charged to develop 750 to 800 V for the seriescombination in order to deliver shocks having a pulse energy of 5 Joulesor more, and more typically 20 Joules or more. In contrast, low voltagepacing pulses delivered using extra-cardiovascular electrodes may beless than 0.1 Joule. The low voltage capacitor(s) charged for deliveringa low voltage pacing pulse may have a capacitance that is much less thanthe HV capacitor, e.g., 6 to 10 microfarads, and may be charged using astate machine to a multiple of the battery charge of power source 90without using a transformer. If the LV capacitor or capacitorcombination is charged to 8 V for a composite pacing pulse amplitude of8 V and total pulse width of 8 ms the delivered energy is approximately1 millijoule if the pacing vector impedance is 500 ohms. Compositepacing pulses, delivered by the LV therapy module 85, having an 8 Vamplitude and 8 ms pulse width may be in the range of 0.5 to 1.3milliJoules for a range of pacing loads between 400 ohms and 1000 ohms.The maximum pulse amplitude available from LV therapy module 85 fordelivering low voltage composite pacing pulses may be 10 Volts in someexamples and may be higher in other examples, for instance not more than40 Volts or not more than 20 Volts. Pacing pulses delivered usingendocardial electrodes or epicardial electrodes may be much lower inenergy, on the order of microjoules, e.g., 2 microJoules to 5microjoules for an endocardial pacing pulse that is 2V in amplitude, 0.5ms in pulse width and applied across a pacing electrode vector impedanceof 400 to 1,000 ohms. An extra-cardiovascular composite pacing pulse maybe greater than 100 microjoules and less than 1 Joule, for example.

Sensor module 92 may include additional sensors for monitoring thepatient for controlling therapy delivery. For example, sensor module 92may include an activity sensor, a posture sensor, a heart sound sensor,or other physiological sensor(s) for monitoring the patient and makingtherapy delivery decisions. In various examples, rate responsive pacingmay be provided based on a patient activity signal. The rate of lowvoltage pacing pulses delivered using extra-cardiovascular electrodesmay be adjusted based on the activity signal. A decision to deliver ATPor shock therapy may be based in part on physiological sensor signals inaddition to the cardiac electrical signal. As such, ATP pulses may bedelivered as LV extra-cardiovascular pacing pulses in response to atherapy delivery decision made by control module 80 using physiologicalsensor signals from sensor module 92.

Control parameters utilized by control module 80 may be programmed intomemory 82 via telemetry module 88. For example, the composite pacingpulse width and pacing pulse amplitude may be programmable parameters.Control module 80 may utilize the programmed pacing pulse width andpacing pulse amplitude for controlling the selection, sequence andcharging of LV capacitors included in LV therapy module 85. Telemetrymodule 88 includes a transceiver and antenna for communicating withexternal device 40 (shown in FIG. 1) using RF communication as describedabove. Under the control of control module 80, telemetry module 88 mayreceive downlink telemetry from and send uplink telemetry to externaldevice 40. IMD 14 may communicate with other implantable devicesimplanted in the patient using telemetry module 88.

FIG. 4A is a depiction of an example of a composite pacing pulse 50 thatmay be generated and delivered by IMD 14 to pace heart 26 usingextra-cardiovascular electrodes, such as an electrode vector selectedfrom among the electrodes 24A, 24B, 28A, 28B and/or 30 and/or housing 15shown in FIGS. 1A-2B. In one example, the composite pacing pulses, suchas pulse 50 of FIG. 4A and pulse 80 of FIG. 4B described below, aredelivered via one of electrodes 28A or 28B as the cathode and thehousing 15 as the return anode. In other examples, the composite pulsesare delivered using one of electrodes 28A or 28B as the cathode and adefibrillation electrode, such as one of defibrillation electrodes 24Aor 24B as the return anode, in examples that include a defibrillationelectrode. The techniques for delivering composite pacing pulsesdisclosed herein, however, are not limited to use with a particularpacing electrode vector.

Composite pacing pulse 50 comprises four pulses 52 a, 52 b, 52 c, and 52d that are each individually delivered by sequentially discharging atleast two different holding capacitors (or two different combinations ofholding capacitors) across a pacing electrode vector via an outputcapacitor. The first pulse 52 a defines a leading edge 58 a of thecomposite pulse 50. Each of the pulses 52 a-52 d has a peak voltageamplitude 66 according to a programmed pulse amplitude. A decayingportion 56 a, 56 b, 56 c, and 56 d of each individual pulse decaysaccording to an RC time constant of the discharge circuit. Eachindividual pulse 52 a-52 d may be truncated at an individual pulse width62. The leading edge 58 b, 58 c and 58 d of the respective pulses 52 b,52 c and 52 d coincides in time with the terminating edge 60 a, 60 b and60 c, respectively of the preceding pulse, 52 a, 52 b, and 52 c,respectively. The terminating edge 60 d of the final pulse 52 d definesthe trailing edge of the composite pulse 50.

The composite pulse 50 has a time-varying pulse amplitude that reaches apeak voltage amplitude 66 at the leading edge 58 a-58 d of eachindividual pulse with periods of decay between the leading edges 58 a-58d to a minimum pulse amplitude 68 just prior to the next leading edge.The individual pulse width 62 may be set to maintain the minimum pulseamplitude 68 of each individual pulse 52 a-52 d above a minimumamplitude threshold to ensure that the total pulse energy delivered inthe composite pulse 50 successfully captures and paces the heart 26. Theindividual pulse width 62 may be fixed, e.g., up to 2 ms in someexamples so that the total pulse width is up to 8 ms when four fused,consecutive pulses 52 a-52 b are delivered as shown in the example ofFIG. 4A. The individual pulse width may be the maximum individual pulsewidth that can be produced by LV therapy module 85 when a single lowvoltage capacitor (or capacitor combination) included in LV therapymodule 85 is discharged to deliver an individual pulse. This maximumavailable pulse width may be based on the effective capacitance of anindividual low voltage, holding capacitor (or combination of holdingcapacitors) and may be a maximum programmable pulse width of anindividual pulse.

The number of individual pulses delivered in fused sequence may beselected based on the total pacing pulse width 64 required to capturethe heart for a given peak voltage amplitude 66 and the individual pulsewidth 62. The composite pacing pulse width 64 may be up to 8 ms (asshown), 10 ms, 12 ms, 16 ms, or even 20 ms or more. In some examples,individual pulse width 62 is set to a maximum individual pulse widththat can be reached without allowing the minimum pulse amplitude 68 tofall to a minimum amplitude threshold. For example, the minimum pulseamplitude 68 may be prevented from reaching 0 V at terminating edges 60a, 60 b, 60 c and 60 d and may be maintained above an amplitudethreshold, which may be defined as a percentage of the programmed peakvoltage amplitude 66, e.g., 25%, 50% or other selected percentage ofprogrammed peak voltage amplitude 66.

In other examples, the pacing pulse amplitude may be monitored real timeduring the delivery of composite pacing pulse 50, and, when the decayingamplitude drops to an amplitude threshold value, the next individualpulse is started. For example, the amplitude of decaying portion 56 amay be sampled, and when the minimum amplitude 68 is reached the nextpulse 52 b is started. The first pulse 52 a is truncated when the nextpulse 52 b is started so that terminating edge 60 a of pulse 52 a andleading edge 58 b of the second pulse 52 b occur simultaneously, ornearly simultaneously within the limits of the electronic circuitry. Itis recognized that limitations within the electronic circuitry mayresult in a non-zero time gap between individual pulses 52 a-52 d insome examples. The delivered energy of each individual pulse 52 a-52 d,however, is fused close enough in time to a preceding and/or subsequentindividual pulse such that the individual pulse energies accumulate toachieve a dose response necessary to achieve capture of the patient'sheart. Each individual pulse 52 a-52 d may have a pulse energy below thecapture threshold of the heart. By delivering the individual pulses 52a-52 b within a time window defined by the total pulse width 64, thetotal composite pacing pulse energy that is delivered is greater thanthe pacing capture threshold of the heart. As such, the composite pulsecaptures the heart even when each individual pulse 52 a-52 d, ifdelivered alone or spaced further apart in time, may be insufficient tocapture and pace the heart. In some examples, a pacing capture thresholdtest may be performed using methods generally disclosed in U.S. Patent.Application No. 62/262,499 and corresponding U.S. Pat. No. 10,080,905,both incorporated herein by reference in their entirety.

Each individual pulse 52 a-52 d may be delivered across the pacingelectrode vector having the same polarity (positive-going in the exampleshown) by sequentially coupling different capacitance elements acrossthe selected pacing electrode vector. Each of the different capacitanceelements are previously charged to the peak voltage amplitude 66 priorto being coupled across the pacing electrode vector. In some examples,the same capacitor or combination of capacitors may not be used todeliver two consecutive individual pulses, e.g., 52 a and 52 b, sincecharging of the capacitor (or combination of capacitors) to the peakvoltage amplitude 66 occurs prior to initiating each respective one ofthe individual pulses 52 a-52 d . The same capacitor or same combinationof capacitors may be used to deliver two non-consecutive individualpulses, e.g., 52 a and 52 d by recharging the same capacitor orcombination of capacitors to the peak voltage amplitude 66 during theintervening one or more individual pulses 52 b and 52 c.

All individual pulses 52 a-52 d are shown to have the same peak voltageamplitude 66 in FIG. 4A. The peak voltage amplitude may be the maximumvoltage amplitude available from the LV therapy module 85 or a maximumvoltage amplitude acceptable by the patient. The total pulse energy ofthe composite pacing pulse 50 is controlled by setting the individualpulse number and individual pulse width of pulses 52 a-52 d. It iscontemplated, however, that one capacitor (or combination of capacitors)that is discharged to deliver one of the individual pulses 52 a-52 d maybe charged to a different voltage than another capacitor (or combinationof capacitors) used to deliver a different one of the individual pulses52 a-52 d. As a result, the individual pulses 52 a-52 d may havedifferent peak voltage amplitudes in some instances. Individual pulses52 a-52 d, however, are generated by switching out a first dischargingcapacitor (or combination of capacitors) and switching in a nextcapacitor (or combination of capacitors) that is(are) charged to thedesired peak voltage amplitude of the next individual pulse. A firstindividual pulse is thereby terminated by stopping discharging of thefirst capacitor(s), and the next individual pulse is started by startingdischarging of the next capacitor(s).

Pacing pulse 50 is followed by a recharge pulse 70 comprising a lowamplitude pulse in opposite polarity for each of the individual pulses52 a-52 d. The recharge pulse 70 may allow an output capacitor of the LVtherapy module 85 to passively discharge if it has charged during thedelivery of pacing pulse 50 to promote charge neutrality. The rechargepulse 70 may reduce polarization artifact of the pacing electrodes.

In FIG. 4A, individual pulses 52 a-52 d are fused in time but are notoverlapping in that the leading and terminating edges of the individualpulses are simultaneous or near simultaneous within the limits of theelectronics. In other examples, the individual pulses may beoverlapping. FIG. 4B is a depiction of a composite pacing pulse 80having leading edge 88 a and terminating edge 90 c according to anotherexample. Pulse 80 includes three overlapping pulses 82 a, 82 b and 82 c.The leading edges 88 b and 88 c of the second and third pulses 82 b and82 c, respectively, occur before the terminating edge 90 a and 90 b ofthe respective preceding pulses 82 a and 82 b. In this case, thedecaying portions 86 a, 86 b and 86 c of the individual pulses 82 a, 82b and 82 c may have differing decay rates due to overlapping portions ofpulse 82 a and pulse 82 b and overlapping portions of pulse 82 b andpulse 82 c. Electronic circuitry such as one or more diodes may be usedto prevent charge distribution between capacitors when an individualpulse 82 b or 82 c is started prior to truncation of the preceding pulse82 a or 82 b, respectively.

FIG. 5 is a schematic diagram of a pacing control module 102 included incontrol module 80 and the LV therapy module 85 included in therapydelivery module 84. LV therapy module 85 includes a capacitor selectionand control module 104 and a capacitor array 110. Capacitor array 110includes multiple holding capacitors 115 a through 115 n (labeled C1through Cn, collectively 115), arranged in parallel in this example.Each capacitor C1 through Cn is selectable via a respective one ofparallel switches 116 a, 116 b, 116 c through 116 n, collectively 116,(labeled Si through Sn). Switches 116 are controlled by capacitorselection and control module 104 to selectively control which capacitorsC1 through Cn 115 are coupled to a pacing pulse output signal line 130via switch 112 for pacing pulse delivery across output capacitor 122.While output capacitor 122 is represented as a single output capacitorelement, it is to be understood that output capacitor 122 may representmultiple output capacitors where one output capacitor may be provided inseries with each holding capacitor 115 a-115 n so that each holdingcapacitor of array 110 can be discharged across a respective outputcapacitor to deliver each individual pulse in timed sequence. In thiscase, each output capacitor may be coupled to a respective holdingcapacitor 115 a-115 n via a respective switch that, when closed, enablesone or more holding capacitors 115 a-115 n selected by closure ofrespective ones of switches 116 to be discharged across the respectiveoutput capacitor. A configuration of LV therapy module 85 includingmultiple output capacitors is shown in FIG. 13.

If output capacitor 122 is provided as a single capacitor as shown, itmay be provided with an equivalent capacitance that is selected based onthe sum of the capacitances of all of the parallel holding capacitorsC1-Cn of array 110 so that the output capacitor 122 does not becomecharged on the first individual pulse in a way that blocks delivery ofsubsequent pulses delivered by subsequent discharging of one or moreholding capacitors 115. For example, output capacitor 122 may have acapacitance at least equal to the sum of capacitors C1-Cn 115.

Switches 116 may be enabled or closed for the individual pulse width oneat a time to couple a respective one of capacitors 115 to output signalline 130 one at a time in a controlled sequence for delivering the fusedindividual pulses of the composite pacing pulse. In some examples,switches 116 may be enabled or closed in combinations that enable two ormore of capacitors 115 a through 115 n to be coupled to output signalline 130 simultaneously for delivering an individual pulse. When two ormore of capacitors 115 a through 115 n are used in combination todeliver an individual pulse of the composite pulse, the higher effectivecapacitance results in a longer RC time constant and slower decay of thepulse amplitude during the composite pacing pulse.

While four capacitors are shown, capacitor array 110 may include more orfewer capacitors, which may depend on the requirements of the particularpacing application and available volume in the housing 15. In otherexamples, capacitor array 110 includes two or three capacitors. In stillanother example, capacitor array 110 includes five or six capacitors.Capacitors C1 through Cn 115 may be provided with a capacitance of 6 to10 microfarads in one example, and may have the same or differentcapacitances, but capacitances greater than or less than this range maybe used to provide a desired effective capacitance for delivering eachindividual pulse. Larger capacitors (or larger effective capacitance ofa combination of capacitors) may enable longer individual pulse widthsto be used to produce an overall longer composite pacing pulse or acomposite pacing pulse comprising fewer individual pulses. C1 through Cncapacitors 115 are shown coupled in parallel. In other examples, somecapacitors in array 110 may be coupled in series, e.g., C1 and C2 may becoupled in series and in parallel with C3. Selectable arrangements ofthe capacitors 115 in parallel and/or in series provides control of theRC time constant of the discharge circuit and the effective capacitanceof the discharge circuit including output capacitor 122 and the pacingelectrode vector impedance according to a particular pacing application.Various configurations of single, parallel or series capacitors 115 maybe selected via switching circuitry included in capacitor array 110 forenabling a desired effective capacitance for delivering an individualpulse of the composite pacing pulse.

Pacing control module 102 provides control signals to capacitorselection and control module 104 to control the timing, pulse amplitude,and pulse width of a composite pacing pulse. The pulse amplitude may beset by pacing control module 102 according to a programmed pulseamplitude, and the pulse width may be controlled according to aprogrammed pulse width. Pacing control module 102 may determine thecapacitor configuration based on the number of fused individual pulsesrequired to achieve the overall pulse width of the composite pacingpulse.

Capacitor selection and control module 104 controls LV charging circuit114 to charge a selected number of the capacitors C1 through Cn 115 fordelivering the composite pacing pulse. For example, three capacitors C1through C3 may be selected one at a time for delivering a compositepulse comprising three fused pulses; four capacitors C1 through C4 maybe selected one at a time for delivering a composite pulse comprisingfour fused pulses and so on. LV charging circuit 114 charges theselected capacitors to a voltage level according to the programmedpacing pulse amplitude to supply pacing pulse energy. Power source 98(FIG. 3) may provide regulated power to LV charging circuit 114. LVcharging circuity 114 may be controlled by a state machine to charge theselected capacitors using a multiple of the battery voltage of powersource 98, e.g., four times the battery voltage. LV charging circuit 114may be controlled to charge the selected capacitors simultaneously orsequentially as needed for delivering the series of fused individualpulses.

In response to a timing signal from pacing control module 102, capacitorselection and control module 104 sequentially couples the selectedcapacitors (or capacitor combinations) one at a time to output signalline 130 via switch 112 to sequentially discharge the selectedcapacitors (or capacitor combinations) across output capacitor 122 and apacing electrode vector coupled to output signal line 130. Eachcapacitor (or combination of capacitors) is discharged for an individualpulse width that is fused in an overlapping or non-overlapping mannerwith the next individual pulse delivered by discharging the nextcapacitor (or combination of capacitors) in the sequence. For example,capacitor selection and control module 104 may sequentially enableselected ones of switches 116 a through 116 n to couple the selectedones of capacitors C1 through Cn 115 to output signal line 130 in asequential order. The capacitor selection and control module 104uncouples capacitor array 110 from output signal line 130 by openingswitch 112 at the expiration of a programmed composite pacing pulsewidth (or upon expiration of the last individual pulse width).

In some examples, LV therapy module 85 includes an analog-to-digitalconverter (ADC) 106 for sampling the pacing pulse output on signal line130 and providing a digital feedback signal to capacitor selection andcontrol module 104 and/or pacing control module 102. During the pacingpulse, pacing control module 102 enables ADC 106 to sample the pacingpulse output signal on output signal line 130 at a desired samplingrate, e.g., every 2 microseconds, throughout the pacing pulse width. ADC106 may be enabled to sample the pacing pulse amplitude from the startof the pacing pulse when switch 112 is enabled until the end of thepacing pulse when switch 112 is disabled or a portion thereof.

Pacing control module 102 may monitor the sampled pacing pulse amplitudereceived from ADC 106 during pacing pulse delivery by comparing thesample points to an amplitude threshold or to an expected amplitude. Anexpected amplitude may be based on the predicted decay rate of anindividual pulse according to a known or estimated RC time constant ofthe discharge circuit. In one example, the sample points are compared toan amplitude threshold, which may be set as a percentage of theprogrammed pacing pulse amplitude, e.g., 50% of the programmed pacingpulse amplitude. If the pacing pulse amplitude falls below the amplitudethreshold, the pacing control module 102 passes a timing signal tocapacitor selection and control module 104 to cause the next individualpulse in the fused series of pulses to begin by switching in the nextcapacitor (or combination of capacitors) in the array 110 to begindischarging (e.g., by closing the associated switch from among switchesS1 through Sn) to maintain the amplitude above a minimum amplitudethreshold level throughout the composite pacing pulse width. Thepreceding individual pulse may continue for a predefined individualpulse width or it may be truncated upon starting the next pulse bydisabling the previously enabled capacitor(s) by opening the associatedswitch(es) from among switches S1 through Sn 116 when the next holdingcapacitor(s) is/are enabled for discharging the next individual pulse.

FIG. 6 is a schematic diagram of pacing control module 102 included incontrol module 80 and capable of accessing instructions stored in memory82 according to one example. Pacing control module 102 may include amicroprocessor 140 that is configured to execute instructions stored inmemory 82 for selecting a series of capacitors or combinations ofcapacitors and setting the pulse amplitude and pulse width fordelivering composite pacing pulses.

Microprocessor 140 provides capacitor configuration data to capacitorconfiguration module 146 which passes the capacitor configuration datato the capacitor selection and control module 104 of LV therapy module85 (FIG. 3). The capacitor (or combination of capacitors) to be selectedfor delivering each individual pulse may be passed to capacitorselection and control module 104 one at a time, e.g., on a clock cycle,until all individual pulses have been delivered. In other examples,capacitor configuration module 146 may pass the capacitor configurationdata for the entire series of individual pulses and capacitor selectionand control module 104 may select the capacitor(s) for each individualpulse in the appropriate, sequential order.

The timing control module 150 may be controlled by microprocessor 140 topass timing signals to capacitor selection and control module 104 tocontrol the timing of the leading edge of the composite pulse, theleading edges of subsequent pulses, the time of truncation of individualpulses, and the terminating edge of the composite pulse. The individualpulse widths and the overall pulse width of the composite pacing pulsemay be predetermined, e.g., stored in memory 82. Microprocessor 140 maycontrol the delivery of the series of fused individual pulses by passingthe capacitor selection and timing information for each individual pulseto capacitor selection and control module 104 via capacitorconfiguration module 146 and timing control module 150.

Microprocessor 140 may also pass instructions to ADC control module 148.ADC control module 148 may be configured to control the sampling rateand sampling intervals over which ADC 106 (FIG. 5) is enabled. Pulseamplitude sample points may be received by data buffer 144 from ADC 106and passed to microprocessor 140. Microprocessor 140 may be configuredto compare the sampled amplitude values to an amplitude threshold. Basedon this comparison, microprocessor 140 may determine when the next pulsein the sequence of individual pulses is required and passes a timingsignal to timing control module 150. Timing control module 150 in turnpasses the timing signal to capacitor selection and control module 104,e.g., on the next clock signal. The capacitor selection and controlmodule 104 responds to the timing signal by switching in the nextcapacitor(s) in the series of capacitors or capacitor combinations thatis scheduled to start the next individual pulse of the series of fusedpulses. For example, the next individual pulse in the series of fusedpulses may be started when the pulse amplitude falls to an amplitudethreshold, below an expected or predicted amplitude based on a known orestimated RC time constant of the discharge circuit, or when apredetermined individual pulse width expires, whichever comes first.

Data buffer 144 may receive impedance data from impedance measurementmodule 90. Microprocessor 140 may retrieve the impedance measurementdata for use in determining the individual pulse width used to deliverfused pulses. The decay rate of an individual pulse will depend on theimpedance of the selected pacing electrode vector and the effectivecapacitance of the individual capacitor or capacitor combination used todeliver the individual pulse. The RC time constant, sometimes referredto as “tau,” may be determined using the measured pacing vectorimpedance and the capacitance of the individual capacitor or capacitorcombination to be used for delivering an individual pulse. The voltageamplitude of the individual pulse at truncation may be predicted atdifferent pulse widths based on the RC time constant. The individualpulse width may be set to a value that results in a predicted voltageamplitude at the terminating edge of the individual pulse that isgreater than an amplitude threshold.

Timing control module 150 receives pacing pulse timing data frommicroprocessor 140, which may include the starting time and/or pulsewidth of each individual pulse. The time to start the first pacing pulseand the pulse width of the individual pulses are passed to capacitorselection and control module 104. The pulse width of the first pulse maybe applied to all individual pulses in the composite pulse such thatupon expiration of the individual pulse width, the nth pulse istruncated and the n+1 pulse is started and an individual pulse widthtimer restarted. If the next pulse in the series of pulses is startedprior to truncation of the preceding pulse, in a fused overlappingmanner as shown in FIG. 4B, the time to start the leading edge of thenext pulse is also passed from timing control module 150 to capacitorselection and control module 104. In other examples, differentindividual pulse widths may be used such that timer control modulepasses individual pulse widths for each pulse in the series. A compositepacing pulse having differing individual fused pulse widths is describedbelow in conjunction with FIG. 14.

FIG. 7 is a schematic diagram of capacitor selection and control module104 included in LV therapy module 85 according to one example. Capacitorselection and control module 104 includes a capacitor control 160,capacitor selection latches 162, pulse width timer 164 and pacing pulseenable/disable gate 166. Capacitor control 160 receives a clock signal154 and an input signal 156 from capacitor configuration control module146 of pacing control module 102 (FIG. 6). The input signal 156 includescapacitor selection data indicating the capacitor(s) that are to beselected for each individual pulse of the composite pacing pulse.

Capacitor control 160 clocks the capacitor configuration data tocapacitor selection latches 162, which store the configuration datauntil passed to the S1-Sn switches 116 of the capacitor array 110 (FIG.5). In accordance with the configuration data, capacitor selectionlatches 162 set separate signals that are passed to each of therespective switches S1-Sn 116 to selectively enable or disable each oneof capacitors C1 through Cn 115 as needed for each individual pulsedelivery. The capacitor selection latches 162 are controlled bycapacitor control 160 to sequentially select individual capacitors orcapacitor combinations to deliver the series of fused pulses.

Pulse width timer 164 receives clock signal 154 and input from timingcontrol module 150 (FIG. 6) on signal line 158. Pulse width timer 164passes a timing control signal to pulse enable/disable gate 166. Forexample, upon expiration of a pacing escape interval, timing controlmodule 150 passes a signal to pulse width timer 164 to enable LV therapymodule 85 to start a pacing pulse. Pulse enable/disable signal gate 166outputs a signal on signal line 168 to switch 112 (FIG. 5) to start thepacing pulse. Switch 112 is controlled by gate 166 to couple the firstselected capacitor or combination of capacitors to pacing pulse outputsignal line 130 for the first individual pulse. Switch 112 remainsenabled or closed for the composite pacing pulse width under the controlof pulse enable/disable gate 166. Pulse width timer 164 may be set tothe composite pulse width so that pulse enable/disable gate 166 disablesor opens switch 112 upon expiration of the composite pacing pulse widthto uncouple the capacitor array 110 from the output signal line 130.

In some examples, capacitor control 160 and pulse width timer 164receive capacitor and timing data from pacing control module 102 onsignal lines 156 and 158 on an individual pulse-by-pulse basis. Pulsewidth timer 164 may enable switch 112 for the duration of the compositepacing pulse width. During the composite pacing pulse width, pulse widthtimer 164 may set an individual pulse width timer for controlling thetimes that capacitor selection and control module 104 enables/disablesindividual switches S1 through Sn 115 according to the capacitorselection for each individual pulse. Pulse width timer 164 may includeone or more individual pulse timers for controlling the termination timeof individual pulses and the timing of the leading edge of the nextpulse. Upon the expiration of each individual pulse width, capacitorcontrol 160 controls capacitor selection latches 162 to select the nextcapacitor or capacitor combination for starting the next individualpulse according to data received on signal line 156. Capacitor control160 may receive data indicating the capacitor(s) to be used for eachindividual pulse in a serial manner on signal line 156 and consecutivelyselect the capacitor selections via latches 162.

Alternatively, all capacitor selection data representing the desirednumber of individual pulses and capacitor selections for delivering theentire composite pacing pulse may be passed to capacitor control 160 ona single pass. Capacitor control 160 sequentially controls latches 162to step through the individual capacitor selections as each individualpulse width expires according to timing signals from timer 164.

In some examples, pacing control module 102 receives the sampled pacingpulse signal amplitude and compares the sampled amplitude to anamplitude threshold. For example, if the pacing pulse amplitude falls toan amplitude threshold before the start of the next individual pulse orexpiration of the composite pulse width, the next capacitor selectionfor the next individual pulse may be triggered.

Upon expiration of the composite pacing pulse width, pulse width timer164 passes a pulse termination signal to pulse enable/disable gate 166that outputs a signal on control signal line 168 that terminates thepacing pulse by disabling switch 112 to uncouple the capacitor array 110from output signal line 130.

FIG. 8 is a flow chart 200 of a method for deliveringextra-cardiovascular pacing pulses by IMD 10 according to one example.At block 202, the composite pacing pulse width is selected. The pacingpulse width may be selected based on capture threshold tests. Forexample, using a default pacing pulse amplitude or a pacing pulseamplitude selected to be below a discomfort level of the patient, theminimum pacing pulse width required to capture (successfully pace) theheart may be determined. The pacing pulse width of the composite pacingpulse may be set to a safety interval longer than the capture pulsewidth threshold to reduce the likelihood of loss of capture.

At block 204 the capacitor sequence for delivering the composite pulseis selected. In one example, the pacing pulse width set at block 202 isdivided by a predetermined individual pulse width to determine thenumber of fused individual pulses having equal pulse widths that isrequired to meet or exceed the composite pacing pulse width. Forexample, if the pulse width capture threshold is 2.5 ms, the compositepacing pulse width may be set to 3 ms at block 202. The predeterminedindividual pulse width may be set to 1 ms so that three fused pulses arerequired to achieve the 3 ms composite pacing pulse width. The capacitorsequence selected at block 204 is three capacitors, e.g., C1, C2 and C3,to be enabled in sequential order for discharging across the pacingvector, each for 1 ms.

In another example, if the composite pacing pulse width is set to 2 ms,the pacing control module 102 may select the capacitor sequence as thecombination of C1 and C2 for delivering the first individual pulse of 1ms and the combination of C3 and C4 to deliver the second individualpulse of 1 ms. In yet another example, if the pacing pulse width is setto 7.5 ms, a sequence of C1-C2-C3-C4-C1 may be selected such that eachindividual capacitor is enabled one at a time in sequence to dischargefor a 1.5 ms individual pulse width, and C1 may be recharged after thefirst individual pulse during the delivery of the second through fourthindividual pulses so that it is enabled to deliver the fifth individualpulse. The five sequential 1.5 ms pulses are fused in a non-overlappingmanner such that a 7.5 ms pacing pulse is delivered.

A maximum composite pacing pulse width may be up to 10 ms or more invarious examples. The maximum individual pulse width may be set based onthe capacitance of the individual capacitor or combination of capacitorsbeing used. For example, the maximum individual pulse width may be 2 ms,4 ms or other predetermined value for a given effective capacitance usedto deliver the individual pulse.

The capacitors 115 of array 110 may be selected singly for deliveringeach individual pulse of the composite pulse or in combinations of twoor more at a time for delivering each individual pulse. Numeroussequential orders of enabling the capacitor(s) selected for deliveringeach individual pulse may be conceived. Furthermore, a selected sequencemay involve one or more capacitors that are discharged, recharged anddischarged again to deliver more than one of the individual pulsesduring a single composite pacing pulse.

The capacitors selected for delivering the composite pacing pulse arecharged by LV charging circuit 114 at block 206 under the control ofcapacitor selection and control module 104. All capacitors that arebeing utilized to deliver the pacing pulse may be charged simultaneouslyto the programmed pacing pulse voltage amplitude. In other examples, thecapacitors may be charged in a sequential order according to the orderin which they will be discharged during pacing pulse delivery. When acapacitor or capacitor combination is being used more than once during acomposite pacing pulse, LV charging circuit 114 recharges the capacitor(or capacitor combination) while another capacitor or capacitorcombination is being discharged to deliver an individual pulse. LVcharging circuit 114 may include capacitor charge pumps or an amplifierfor the charge source to enable rapid recharging of holding capacitorsincluded in capacitor array 110.

At block 208, pacing control module 102 determines if it is time fordelivering a pacing pulse. This determination may be made based on theexpiration of a pacing interval. The pacing interval may be, for exampleand with no limitation intended, a V-V pacing escape interval such as alower rate interval for bradycardia pacing, a back-up escape intervalfor pacing during asystole or post-shock pacing, or an ATP intervalduring delivery of ATP. The pacing interval may alternatively be aninterval used to deliver entrainment pacing pulses prior to T-shockdelivery for tachyarrhythmia induction or a 50 Hz burst interval fortachyarrhythmia induction. Pacing control module 102 may wait for apacing escape interval to expire at block 208, and, when it expires,timing control module 150 passes a signal to capacitor selection andcontrol module 104 to start a composite pacing pulse.

Capacitor selection and control module 104 may set a composite pulsewidth timer included in pulse width timer 164 at block 210 and couplesthe capacitor array 110 to pacing output signal line 130, e.g., byenabling or closing switch 112 of FIG. 5. At block 212, capacitorselection and control module 104 sequentially enables the capacitor(s)selected for delivering each individual pulse according to data receivedfrom capacitor configuration module 146 (FIG. 6). Each capacitor orcombination of capacitors in the series is enabled by closing respectiveswitch(es) S1-Sn 116 for the individual pulse width to discharge theselected capacitor(s) across the pacing electrode vector. A singlecapacitor or combination of capacitors is selected as a capacitanceelement that is discharged for delivering of an individual pulse of thecomposite pacing pulse. Pacing control module 102 selects a sequence ofcapacitance elements from the capacitors of the capacitor array 110 anda respective individual pulse width for each capacitance element of thesequence so that a sum of the individual pulse widths is equal to orgreater than a selected composite pacing pulse width, which is equal toor greater than a pacing pulse width capture threshold for the pacingpulse voltage amplitude being used.

Upon completion of the sequence of fused pulses at the expiration of thecomposite pacing pulse width, capacitor selection and control module 104uncouples capacitor array 110 from the pacing output signal line 130 atblock 214, and the composite pacing pulse is complete. The compositepacing pulse is delivered to evoke a single depolarization of a heartchamber, e.g., a ventricular heart chamber, to cause a single mechanicalcontraction or beat of the heart chamber. The leading edge pulseamplitude of each individual pulse and the composite pulse width areselected so that the cumulative delivered energy of the fused individualpulses meets or exceeds the pacing capture threshold using theextra-cardiovascular pacing electrode vector selected to pace the heart.Each individual pulse of the composite pulse may have a pulse energybelow the capture threshold but the combined individual pulse energiesaccumulate during the composite pulse width to reach the pacing capturethreshold to cause an evoked response.

FIG. 9 is a flow chart 250 of a method that may be performed by IMD 14for selecting a capacitor sequence, e.g., at block 204 of FIG. 8. Atblock 252, the pacing control module 102 sets an amplitude threshold.The amplitude threshold may be at or above a minimum acceptable voltageof the decaying individual pulses included in the composite pacingpulse. The amplitude threshold may be set based on the programmed pacingpulse amplitude, e.g., 50% of the programmed pacing pulse amplitude. Atblock 254, the pacing control module 102 acquires an impedancemeasurement of the pacing electrode vector. Control module 80 maycontrol impedance measurement module 90 to perform an impedancemeasurement of the pacing electrode vector, or pacing control module 102may retrieve a previous impedance measurement stored in memory 82.

At block 256, the pacing control module 102 selects an elementcapacitance. The element capacitance is the effective capacitance of asingle capacitor or a combination of two or more capacitors of capacitorarray 110 that may be selected simultaneously in series and/or inparallel for delivering an individual pulse of the composite pacingpulse. In some examples, the element capacitance selected at block 256is the capacitance of a single holding capacitor 115 a, 115 b, 115 c or115 n in capacitor array 110 or the effective capacitance of a singleholding capacitor 115 a-115 n and a respective output capacitor 122. Inother examples the element capacitance selected at block 256 depends onthe impedance measurement. If the pacing electrode vector impedancemeasurement is relatively low, a higher element capacitance may beselected, e.g., two of capacitors 115 in parallel. If the pacingelectrode vector impedance is relatively high, pacing control module 102may select an element capacitance of a single capacitor of capacitors115.

At block 258, pacing control module102 determines the RC time constantfor the measured pacing electrode vector impedance and selected elementcapacitance. Based on the RC time constant, the pacing control module102 may predict a maximum possible individual pulse width that could bedelivered without the pulse amplitude falling below the amplitudethreshold at the terminating edge of the individual pulse, before theleading edge of the next pulse in the sequence. For example, the maximumindividual pulse width may be estimated as the time expected for theindividual pulse to decay from the programmed pacing pulse amplitude atthe pulse leading edge to the amplitude threshold based on the RC timeconstant. At block 260, the maximum possible individual pulse width isdetermined for each element capacitance for the series of individualpacing pulses. Pacing control module 102 may set the actual individualpulse width for each individual pulse at block 262 to the maximumpossible individual pulse width or less than the maximum possibleindividual pulse width.

At block 264, pacing control module 102 determines the number ofindividual pulses at the individual pulse width that are required to atleast reach the composite pacing pulse width. For example, if thecomposite pacing pulse width is set to 4 ms, and the individual pulsewidth is set to 1 ms at block 262 based on the RC time constant, fourfused pulses are required. If the individual pulse width is set to 0.75ms, the minimum number of pulses may be determined to be 6 pulses whichproduce a total individual pulse duration of 4.5 ms, longer than thecomposite pacing pulse width. At block 214 of FIG. 8, the capacitorarray 110 may be uncoupled from the pacing output signal line 130 at 4.0ms, truncating the last individual pulse 0.25 ms after its leading edge.In other examples, capacitor array 110 may be uncoupled from the pacingoutput signal line 130 after all individual pulses are delivered for thefull individual pulse width such that the composite pacing pulse widthis exceeded by a portion of an individual pulse width in some cases.

At block 266, the capacitor elements of the capacitor sequence areselected based on the number of individual pulses required and theelement capacitance. To illustrate, if the minimum number of individualpulses was determined to be four based on the element capacitance of 10microfarads (each capacitor C1 through Cn being 10 microfaradcapacitors), the capacitor element sequence may be selected asC1-C2-C3-C4 at block 266. If only three capacitors are available, thecapacitor sequence may be selected as C1-C2-C3-C1. If the elementcapacitance is selected as twice 10 microfarads for delivering two fusedpulses each of 4 ms long for a composite pulse width of 8 ms, thecapacitor elements of the sequence may be selected as C1 in parallelwith C2 for the first element and C3 in parallel with C4 for the secondelement at block 266. After selecting the capacitor elements of thecapacitor sequence at block 266, the capacitors included in the sequencemay be charged at block 206 of FIG. 8 in preparation for delivering thepacing pulse.

In some cases a relatively larger effective capacitance may be selectedfor the first individual pulse, either as a single larger capacitor or acombination of capacitors, to ensure that the decay rate of the firstpulse is not faster than expected based on feedback from ADC 106.Subsequent capacitance elements may be selected in real-time by pacingcontrol module 102 based on the decay rate of the first pulse. If thedecay rate is faster than expected, subsequent capacitance elements maybe selected to have an equal or higher effective capacitance than thefirst capacitance element. If the first pulse does not decay faster thanexpected, remaining individual pulses may be delivered with a lowereffective capacitance than the first pulse. A larger capacitance elementmay be selected for the first individual pulse as a single holdingcapacitor when one capacitor, e.g., C1 115 a, has a larger capacitancethan the other capacitors C2-Cn 115 b-115 n. Alternatively, a largercapacitance element may be selected for the first individual pulse byselecting two or more individual holding capacitors in parallel, forexample C1 115 a and C2 115 b in parallel.

FIG. 10 is a flow chart 300 of a method for delivering a compositepacing pulse according to another example. At block 302, a capacitorsequence is selected. The capacitor sequence includes capacitor elements(one capacitor or a combination of capacitors) that are selected in asequential order for delivering the sequence of individual pulses. Thecapacitor sequence may be selected according to the methods described inconjunction with FIGS. 8 and 9 or may be predefined.

At block 304, the capacitor array 110 is charged according to aprogrammed pacing pulse amplitude. At block 306, the pacing controlmodule 102 passes a timing signal to capacitor selection and controlmodule 104 at the time pacing pulse delivery is needed. At block 308,the capacitor selection and control module 104 sets the composite pulsewidth timer and couples the capacitor array 110 to the pacing outputsignal line 130 for the composite pacing pulse width.

At block 310, an individual pulse is started by enabling the firstcapacitor element of the capacitor sequence by closing associatedswitch(es) 116 to allow the first capacitor element to discharge acrossthe pacing electrode vector. During the pulse, the pacing control module102 may enable ADC 106 to monitor the pulse amplitude at block 312.Pacing control module 102 receives the sampled voltage signals andcompares them to an amplitude threshold at block 314. Pacing controlmodule 102 may establish the amplitude threshold based on the programmedpulse amplitude, e.g., 50% or another percentage of the programmed pulseamplitude. If the sampled pulse amplitude is less than the amplitudethreshold at block 314, the next individual pulse is started at block310.

If the sampled pulse amplitude remains above the threshold at block 314and the individual pulse width timer expires at block 316, the nextindividual pulse is started at block 310 by selecting the next capacitorelement in the sequence. The pulse amplitude may continue to be sampledand monitored as long as the pulse amplitude remains above the amplitudethreshold, until the individual pulse width expires at block 316. If theindividual pulse width timer expires at block 316, and the compositepulse width timer has not expired (block 318), the next individual pulseis started at block 310. If the composite pulse width timer has expired,the capacitor array 110 is uncoupled from the output signal line 130 atblock 320, and the delivery of the pacing pulse is complete. The processmay return to block 304 to charge the capacitors 115 and wait for thetime the next pacing pulse is needed at block 306.

In some examples, expiration of the composite pulse width timer at anytime during an individual pulse (e.g., before expiration of theindividual pulse width timer) may cause truncation of the last pulsebefore expiration of the individual pulse width timer. In otherinstances, upon expiration of the composite pulse width timer, theindividual pulse being delivered may be allowed to continue until theindividual pulse width expires, which may be after expiration of thecomposite pulse width timer. Using the method of FIG. 10, the number ofpulses in the composite pulse and/or the individual pulse width may ormay not be predetermined by pacing control module 102. The pacingcontrol module 102 may cause the capacitor selection and control module104 to enable the next capacitor element of a sequence to start the nextindividual pulse (n+1) when the sampled pulse amplitude of the currentpulse (nth pulse) falls to or below an amplitude threshold. Individualpulses may continue to be delivered in this manner until the compositepulse width timer expires without determining the number of fused pulsesin advance of delivering the composite pacing pulse. In this case, thecapacitor sequence selected at block 302 may include a sequence ofcapacitor elements that may be used to deliver up to a maximum number ofindividual pulses, e.g., 10 pulses, but not all of the capacitorelements in the sequence may be used to deliver pulses if the compositepulse width timer expires before the maximum number of individual pulsesis reached.

FIG. 11 is a conceptual diagram of IMD 14 coupled to transvenous leadsin communication with the right atrium (RA) 402, right ventricle (RV)404 and left ventricle 406 of heart 26. In some examples, IMD 14 is amulti-purpose device that can be programmably configured to operate as amulti-chamber pacemaker and defibrillator when coupled to transvenousleads 410, 420 and 430 or as an extra-cardiovascular pacemaker anddefibrillator when coupled to one or more extra-cardiovascular lead(s),e.g., lead 16 as shown in FIGS. 1A-2B. IMD 14 is shown implanted in aright pectoral position in FIG. 11; however it is recognized that IMD 14may be implanted in a left pectoral position, particularly when IMD 14includes cardioversion and defibrillation capabilities using housing 15as an electrode.

In the example of FIG. 1A, IMD 14 may have a connector assembly 17having a single connector bore for receiving extra-cardiovascular lead16. In the configuration of FIG. 11, IMD 14 includes connector assembly170 having three connector bores for receiving proximal connectors ofright atrial (RA) lead 410, right ventricular (RV) lead 420, andcoronary sinus (CS) lead 430 to enable IMD 14 to deliver multi-chamberpacing to heart 26. RA lead 410 may carry a distal tip electrode 412 andring electrode 414 for obtaining atrial intra-cardiac electrogram (EGM)signals and delivering RA pacing pulses. RV lead 420 may carry pacingand sensing electrodes 422 and 424 for obtaining an RV EGM signal anddelivering RV pacing pulses. RV lead 420 may also carry RVdefibrillation electrode 426 and a superior vena cava (SVC)defibrillation electrode 428. Defibrillation electrodes 426 and 428 areshown as coil electrodes spaced apart proximally from the distal pacingand sensing electrodes 422 and 424.

CS lead 430 is shown as a quadripolar lead carrying four electrodes 432that may be positioned along a cardiac vein 405. CS lead 430 may beadvanced through the coronary sinus into a cardiac vein 405 to positionelectrodes 432 along the left ventricular lateral wall for obtaining aleft ventricular EGM signal and for delivering pacing pulses to the leftventricle. In other examples, one or more electrodes carried by CS lead430 may be positioned along left atrium 408 for obtaining left atrialEGM signals and/or pacing the left atrium 408.

IMD 14 may be configured to provide dual chamber or multi-chamber pacingtherapies, including CRT, using the electrodes 412, 414, 422, 424 and432 of transvenous leads 410, 420 and 430. IMD 14 may also be capable ofdetecting and discriminating cardiac tachyarrhythmias and deliveringCV/DF shocks as needed using defibrillation electrodes 426 and 428.

FIG. 12A is a conceptual diagram of IMD 14 and a proximal portion ofextra-cardiovascular lead 16 of FIG. 2A or 2B. IMD connector assembly170 includes three connector bores 440, 450 and 460. Connector bore 440may include four electrical contacts 442, 444, 446 and 448 and mayconform to the DF-4 industry standard. The electrical contacts 442, 444,446 and 448 are electrically coupled to electronics enclosed withinhousing 15 via electrical feedthroughs extending from connector assembly170 into housing 15.

Extra-cardiovascular lead 16 includes a proximal lead connector 470which may be a quadripolar, in-line connector and may conform to theDF-4 industry standard. Lead connector 470 is configured to mate withconnector bore 440 and may include a pin terminal 472 and three ringterminals 474, 476 and 478, which are configured to mate withcorresponding contacts 442, 444, 446, and 448, respectively, alignedalong connector bore 440. Contacts 442 and 444 may be coupled to HVtherapy module 83 and are therefore I-TV contacts which becomeelectrically coupled to at least one defibrillation electrode carried bylead 16, e.g., electrodes 24A and 24B shown in FIG. 2A or electrodes24A′ and 24B′ shown in FIG. 2B.

Ring terminals 476 and 478 of lead connector 470 may each be coupled topacing and sensing electrodes 28A and 28B (or 28A′ and 28B′) of lead 16via necessary conductors (not shown) extending through lead body 18 (or18′). Ring terminals 476 and 478 are configured to mate with respectivecontacts 446 and 448 of connector bore 440 so that electrodes 28A and28B (or 28A40 and 28B′) are electrically coupled to LV therapy module585 via terminals 476 and 478 and contacts 446 and 448. As describedbelow in conjunction with FIG. 13, LV therapy module 585 is programmablyconfigurable to either operate to deliver composite pacing pulses forextra-cardiovascular pacing when IMD 14 is coupled to anextra-cardiovascular lead 16 or to operate to deliver multi-channel,multi-chamber pacing when IMD 14 is coupled to a set of transvenousleads such as leads 410, 420 and 430. While not shown in FIG. 12A, it isunderstood that additional connections may exist as needed betweencontacts 442, 444, 446 and 448 and other circuitry within housing 15,such as electrical sensing module 86 and impedance measurement module 90(both shown in FIG. 3) and for coupling all electrodes 24A, 24B, 28A and28B to LV therapy module 585 so that a pacing electrode vector may beselected from any combination of electrodes 24A, 24B, 28A and 28B (or28A′ and 28B′) and/or housing 15.

When IMD 14 is coupled to extra-cardiovascular lead 16, bores 450 and460 may be unused. These bores 450 and 460 may be sealed with plugs 480and 482. As described below, IMD 14 may automatically configure LVtherapy module 585 for delivering low voltage composite pacing pulsesvia contacts 442, 444, 446 and/or 448 by coupling capacitor array 110 tocontacts 442, 444, 446 and/or 448 in a manner that allows selectedcapacitors or combinations of capacitors to be sequentially coupled to442, 444, 446 and/or 448 and discharged across a pacing vector selectedfrom among pacing electrodes 24A, 24B, 28A and 28B (or 28A′ and 28B′)and/or housing 15. In some examples, electrical contacts 442 and 444 ofconnector bore 440 may also be coupled to LV therapy module 585 so thatdefibrillation electrodes or defibrillation electrode segments carriedby lead 16 may be selected in a pacing electrode vector for delivery ofextra-cardiovascular pacing pulses produced by LV therapy module 585.For example, a pacing electrode vector may be selected to includepace/sense electrode 28A or 28B (or 28A′ or 28B′) as the pacing cathodeand a defibrillation electrode 24A or defibrillation electrode 24B (or24A′ or 24B′) as shown in FIG. 2A (or FIG. 2B) as the return anodeelectrode.

FIG. 12B is a conceptual diagram of IMD 14 and proximal portions of eachof transvenous leads 410, 420 and 430 of FIG. 11, which carry therespective electrodes 412, 414, 422, 424, 426, 428, and 430 shown inFIG. 11. In this example, RV lead 420 includes an in-line quadripolarlead connector 425 configured to mate with connector bore 440, which mayconform to the DF-4 industry standard. RV lead connector 425 is similarto extra-cardiovascular lead connector 470 described above, including apin terminal 492 and three ring terminals 494, 496, and 498 configuredto mate with corresponding contacts 442, 444, 446 and 448, respectively,of connector bore 440.

Pin terminal 492 and ring terminal 494 are electrically coupled to RVdefibrillation electrode 426 and SVC defibrillation electrode 428 viaelongated electrical connectors (not shown) extending through lead 420.Pin terminal 492 and ring terminal 494 are electrically coupled to HVtherapy module 83 via contacts 442 and 444 when RV lead connector 425 isproperly seated within connector bore 440 to enable HV therapy, e.g.,CV/DF shocks, to be delivered via defibrillation electrodes 426 and/or428. Ring terminals 496 and 498 are coupled to LV therapy module 585when RV lead connector 425 is properly seated in connector bore 440,thereby electrically coupling RV pacing and sensing electrodes 422 and424 (electrically coupled to terminals 496 and 498 via respectiveconductors extending through lead 420) to LV therapy module 585 viacontacts 446 and 448.

RA lead 410 includes a proximal connector 415 having terminals 416,which may conform to the IS-1 industry standard, for mating withconnector bore 460. Connector bore 460 includes a pair of contacts thatmake electrical contact with RA lead connector terminals 416 when RAlead connector 415 is properly seated in connector bore 460, therebyproviding electrical connection between RA electrodes 412 and 414 (whichare electrically coupled to terminals 416 via respective conductorsextending through lead 410) and LV therapy module 585 for delivering RApacing pulses.

CS lead 430 includes proximal connector 435, shown as an in-line,quadripolar connector having four connector terminals 436, which mayconform to the IS-4 industry standard. When CS lead connector 435 isproperly seated within connector bore 450, the electrodes 432 of CS lead430 (FIG. 11) are electrically coupled to LV therapy module 585 viaelectrical connection between the CS lead connector terminals 436 andrespective contacts of connector bore 450. It is understood thatadditional connections between the contacts of connector bores 440, 450and 460 and other internal IMD circuitry such as electrical sensingmodule 86 and impedance measurement module 90 of FIG. 3 may be providedas needed but are not shown in FIG. 12B for the sake of clarity.

FIG. 13 is a conceptual diagram of LV therapy module 585 when IMD 14 isprogrammably configurable as a multi-channel pacing device for use withtransvenous leads and electrodes (as shown in FIGS. 11 and 12 b) or anextra-cardiovascular pacing device for use with an extra-cardiovascularlead and electrodes (as shown in the examples of FIGS. 1A-2C and FIG.12A). Capacitor array 610 of LV therapy module 585 includes three pacingchannels 602, 604 and 606 that may provide at least three separatepacing outputs when IMD 14 is programmed to operate as a multi-channel,multi-chamber pacemaker. For example, when IMD 14 is coupled to threetransvenous leads 410, 420 and 430 as shown in FIG. 12B, the threepacing channels 602, 604 and 606 may be referred to as the leftventricular output channel 602 coupled to coronary sinus lead 430; theright ventricular output channel 604 coupled to RV lead 420, and theatrial output channel 606 coupled to RA lead 410. The three pacingchannels 602, 604 and 606 are disconnected from one another by openingor disabling operation configuration switches 620 a-620 d (collectively620) and 630 such that each channel 602, 604 and 606 delivers pacingpulses to a respective heart chamber along separate signal pacing outputlines 642, 646 and 648, respectively, for delivering multi-chamberintra-cardiac pacing pulses.

IMD 14 may be programmed, however, to operate as a single channelextra-cardiovascular pacemaker. When programmed to operate as anextra-cardiovascular pacemaker the three pacing channels 602, 604 and606 are tied together by closing operation configuration switches 620a-620 d and 630 such that signal pacing output line 646 is used todeliver extra-cardiovascular pacing pulses, e.g., viaextra-cardiovascular lead 16 as shown in the examples of FIGS. 1A-2B andFIG. 12A, when it is coupled to IMD 14 via connector bore 440.

Capacitor array 610 includes an array of four holding capacitors 612,614, 616 and 618 which may be coupled to the three separate pacingchannels 602, 604 and 606 when operation configuration switches 620 and630 are open to enable multi-channel pacing. The control of capacitorarray 610 by capacitor selection and control module 504 when IMD 14 isprogrammed to operate as a multi-channel pacemaker will be describedfirst.

Beginning with pacing channel 602, a holding capacitor 612 and a back-upholding capacitor 614 are charged or topped off by LV charging circuit514 during time intervals between left ventricular pacing pulses.Holding capacitor 612 may be selectively coupled to one of outputcapacitors 632 a, 632 b, 632 c, and 632 d via pace enable switch 622 anda selected one of respective cathode electrode selection switches 634 a,634 b, 634 c and 634 d. Output lines 642 a, 642 b, 642 c and 642 d arecoupled to respective ones of electrodes 436 carried by quadripolarcoronary sinus lead 430 when lead 430 is connected to IMD 14 viaconnector bore 450 shown in FIG. 12B.

Electrode selection switches 634 a-634 d select which of the outputcapacitors 632 a-632 d of respective output signal lines 642 a-642 d iscoupled to holding capacitor 612 for delivering a pacing pulse. Anotherone of electrodes 432 carried by coronary sinus lead 430 may be selectedas a return anode electrode and coupled to ground. Capacitor selectionand control module 504 controls switch 622 and one of electrodeselection switches 634 a through 634 d to be closed for the duration ofa left ventricular pacing pulse being delivered using coronary sinuslead 430. Back-up capacitor 614 is coupled to the selected outputcapacitor 632 a, 632 b, 632 c or 632 d via switch 624 when a back-upventricular pacing pulse is needed, e.g., due to loss of capturedetection.

Pacing channel 604 may be used for delivering right ventricular pacingpulses using RV lead 420. Holding capacitor 616 is coupled to outputcapacitor 636 along output line 646 when pace enable switch 626 isclosed by capacitor selection and control module 504 according to RVpacing pulse timing information.

Pacing channel 606 may be used for delivering right atrial pacing pulsesusing RA lead 410. Holding capacitor 618 is discharged through outputcapacitor 638 on output line 648 when pace enable switch 628 is closedunder the control of capacitor selection and control module 504according to RA pacing pulse timing information received from pacingcontrol module 102.

If IMD 14 is programmed to operate as an extra-cardiovascular pacemaker,capacitor selection and control module 504 closes operationconfiguration switches 620 and 630 so that all holding capacitors 612,614, 616 and 618 can be discharged to output line 646 at appropriatetimes via respective output capacitors 632 a-632 d, 636 and 638. Asshown in the example of FIG. 12A, when IMD 14 is used for deliveringpacing pulses using extra-cardiovascular electrodes carried by lead 470,connector bores 450 and 460 may be sealed by plugs 480 and 482 such thatall pacing current is directed to output line 646.

When programmed to deliver extra-cardiovascular pacing pulses, pacingchannel 602 may be used to deliver one or more individual pulses of acomposite pacing pulse by selectively closing electrode selectionswitches 634 a-634 d, one at a time or collectively at the same time, todischarge one of holding capacitors 612 or 614 or the combination ofcapacitors 612 and 614 across respective output capacitors 632 a-632 bto output line 646. If pacing channel 604 is selected to deliver anindividual pulse, switch 626 is closed to discharge capacitor 616 acrossoutput capacitor 636. Likewise, if pacing channel 606 is selected todeliver an individual pulse of a composite pacing pulse, switch 628 isclosed to discharge capacitor 618 across output capacitor 638 to outputline 646.

In one example, holding capacitors 612, 614, 616 and 618 are 10microfarad capacitors. Output capacitors 632 a-632 d, 636, and 638 areeach 7 microfarad capacitors. One holding capacitor 612, 614, 616, or618 in series with and one respective one output capacitor 632 a, 632 b,632 c,632 d, 636 or 638 has an effective capacitance of 4 microfarads.The maximum available pulse width for the effective capacitance of 4microfarads may be set to 2 ms. Accordingly, an individual pulse may bedelivered by discharging one holding capacitor 612 or 614 or 616 or 618for up to 2 ms across one of output capacitors 632, 636 or 638,respectively.

If holding capacitor 612 and back-up holding capacitor 614 are selectedin parallel by closing both of pace enable switches 622 and 624, and aredischarged across all of the parallel output capacitors 632 a-632 d byclosing all of selection switches 634 a-634 d, the effective capacitanceis 12 microfarads in the example given above of each of holdingcapacitors 612 and 614 being 10 microfarads and output capacitors 632a-632 d each being 7 microfarads. The maximum available individual pulsewidth may be set to 4 ms for this effective capacitance. The highereffective capacitance results in a longer RC time constant such that themaximum possible individual pulse width is longer than when the holdingcapacitors 612, 614, 616 or 618 are selected one at a time with onerespective output capacitor selected from capacitors 632, 636 or 638.

Capacitor selection and control module 504 selects which holdingcapacitors 612, 614, 616 and 618 are coupled to output line 646 and inwhat combinations and sequence by controlling respective switches 622,624, 626 and 628 and electrode selection switches 634 a-634 d of pacingchannel 602. A sequence of pulses may be delivered to produce acomposite pacing pulse by sequentially discharging holding capacitors612, 614, 616 and 618 one at a time (or one combination at a time)across respective output capacitors 632 a-d, 636 or 638 by sequentiallyenabling or closing the respective switches 622, 624, 626, 628. Forexample, at least two of holding capacitors 612, 614, 616 and 618 aresequentially discharged to produce a composite pacing pulse of at leasttwo fused individual pulses.

Referring again to the example of FIG. 12A, output line 646 may beelectrically coupled to a pacing cathode electrode carried by lead 470via ring terminal 476, and a return anode electrode carried by lead 470may be coupled to ground via ring terminal 478 duringextra-cardiovascular pacing. The pacing cathode electrode and returnanode electrode may correspond to electrodes 28A and 28B, respectively,as shown in FIG. 1A, for example, or any pacing electrode vectorselected from among electrodes 24A, 24B, 28A, 28B, 30 and housing 15. Inother examples, with reference to FIG. 2A (or 2B), one of electrodes 28Aor 28B (or 28A′ or 28B′) may be selected as the pacing cathode and oneof the defibrillation electrodes 24A or 24B (or 24A′ or 24B′) may beselected as the return anode.

In other examples, two pacing channels, e.g., channel 602 and 604, maybe coupled together to output line 646 by enabling or closing switches620 to enable sequential fused pulses to be delivered using holdingcapacitors 622, 624 and 626. Composite pacing pulses including at leasttwo fused individual pulses may be delivered using the two channels 602and 604 electrically coupled to output line 646. The third pacingchannel 606 may be isolated from output line 646 by disabling or openingoperation configuration switch 630. The third pacing channel, pacingchannel 606 in this example, may remain separate and available for otherpacing purposes. Alternatively, channels 604 and 606 may be coupled tooutput line 646 by enabling or closing operation configuration switch630 and opening switches 620. In this case pacing channel 602 remainsseparate and available for other pacing purposes and channels 604 and606 are tied together for delivering composite pacing pulses.

FIG. 14 is a conceptual diagram of one example of a composite pacingpulse 650 that may be delivered by LV therapy module 585 according totechniques disclosed herein. In this example, individual pulses 652, 654and 656 have differing pulse widths. With reference to FIG. 13, pulse652 may be delivered by discharging holding capacitor 616 across outputcapacitor 636 on output line 646 for an individual pulse width 662.Pulse 654 may be delivered by discharging holding capacitor 618 acrossoutput capacitor 638 to output line 646 for individual pulse width 664.In the illustrative example given above, when the effective capacitanceof one holding capacitor 616 or 618 and the respective output capacitor636 or 638 is 4 microfarads, the pulses 652 and 654 may each be 2 ms inpulse width.

The last individual pulse 656 has a pulse width 666 that is longer thanpulse widths 662 and 664 and may be delivered using a larger effectivecapacitance than the capacitance used to deliver pulses 652 and 654.Continuing the illustrative example given above, if parallel 10microfarad holding capacitors 612 and 614 are used to deliver pulse 656across all of the 7 microfarad output capacitors 632 a-632 d inparallel, the effective capacitance is 12 microfarads. The pulse width666 may be set to 4 ms, longer than pulse widths 662 and 664. Thecomposite pacing pulse width 670 is 8 ms in this example.

The leading pulse amplitude 660 of each pulse 652, 654 and 656 may beprogrammable to any of a range of pulse amplitudes, e.g., 1 V, 2 V, 4 V,6 V, and 8 V. The pulse amplitude 660 may be selected to be greater thanthe pacing amplitude capture threshold when the composite pulse width670 is 8 ms. A non-zero gap between each pulse 652, 654 and 656 mayoccur due to limitations of the electronics, but pulses 652, 654 and 656are delivered close enough in time to provide a cumulative deliveredpulse energy within the composite pacing pulse width 670 that is greaterthan the pacing capture threshold even when each pulse 652, 654 and 656individually have a pulse energy that is less than the pacing capturethreshold of the patient's heart.

In other examples, longer pulse 656 may be delivered first with one ormore shorter pulses 652 and 654 following, or longer pulse 656 may bedelivered between shorter pulses 652 and 654. It is recognized thatnumerous combinations of individual pulse number, individual pulsewidths and individual pulse sequences can be conceived for delivering acomposite pacing pulse utilizing varying effective capacitances for eachindividual pulse selected from a capacitor array including multipleholding and output capacitors, which may have differing capacitancevalues, without departing from the scope of the extra-cardiovascularpacing techniques disclosed herein. Negative-going recharge pulses arenot shown in FIG. 14 but it is to be understood that composite pacingpulse 650 may be a biphasic composite pacing pulse having anegative-going portion similar to that of composite pacing pulse 50 ofFIG. 4A having recharge pulses 70.

FIG. 15 is a flow chart 700 of a method for programmably configuring IMD14 to operate as either a multi-channel, multi-chamber pacemaker inconjunction with transvenous leads or as a single-channel pacemaker inconjunction with an extra-cardiovascular lead and extra-cardiovascularelectrodes. If control module 80 receives a user command via telemetrymodule 88 (FIG. 3) indicating that extra-cardiovascular pacing should beenabled, as determined at block 702, the capacitor array 610 (FIG. 13)is configured to enable sequential pacing pulses on a single output line646. At block 704, control module 80 sends control signals to capacitorselection and control module 504 (FIG. 13) to enable or close operationconfiguration switches 620 and 630 so that all holding capacitors 612,614, 616 and 618 can be selectively coupled to output line 646 atappropriate times for discharging across an extra-cardiovascular pacingelectrode vector coupled to output line 646. Output lines 642 a-642 dand 648 are tied to output line 646 by holding operation configurationswitches 620 a-620 d and 630 in closed or enabled states at block 704.

If extra-cardiovascular pacing is not enabled by a user command at block702, control module 80 controls capacitor selection and control module504 to hold switches 620 a-620 d and 630 in an open or disabled state sothat holding capacitors 612, 614 and 618 cannot be coupled to outputline 646 during pacing. Electrode selection switches 634 a-634 d areselectively opened or closed to enable pacing channel 602 to deliverpacing pulses to electrode(s) coupled to respective output lines 642a-642 d. When operation configuration switches 620 a-620 d and 630 areheld in an open state, pacing channel 606 is enabled for pacing pulsedelivery on output line 648 using holding capacitor 618 and outputcapacitor 638. Pacing channel 604 is enabled for pacing pulse deliveryon output line 646 using holding capacitor 616 and output capacitor 636.

In some examples, control module 80 automatically determines whetherextra-cardiovascular pacing should be enabled at block 702 based uponautomatic detection of electrodes coupled to connector bores 440, 450and 460 rather than based on a user-entered command. Automatic detectionof electrodes coupled to connector bores 440, 450 and 460 may be basedon impedance measurements by impedance measurement module 90. Whenimpedance measurements are high indicating an open circuit conditionacross connectors included in connector bores 450 and 460, anextra-cardiovascular pacing configuration is enabled at block 704. Whenimpedance measurements are relatively lower indicating connection of alead and electrodes within connector bores 450 and 460, multi-channelpacing configuration is enabled at block 706.

The operation configuration switches 620 a-620 d and 630 may be set asingle time to a closed or enabled state for single channelextra-cardiovascular pacing or to an open or disabled state formulti-channel pacing and remain in that state for the duration of theoperational life of IMD 14. The operation configuration of IMD 14 may beset at the time of manufacture or set by a user based on the intendeduse of IMD 14. It may be assumed that IMD 14 will be used only asmulti-channel pacemaker with transvenous leads or only as asingle-channel extra-cardiovascular pacemaker for the duration of itsuseful life. In other examples, if a patient's therapeutic needs change,the lead(s) and electrodes coupled to IMD 14 may be removed and replacedwith a different lead(s) and electrodes and the operationalconfiguration of IMD 14 may be programmably (manually or automatically)changed as needed. For example, single channel extra-cardiovascularpacing may be adequate for a patient initially and at a later timemulti-channel pacing may be required for providing multi-chamber pacingtherapy due to a change in the patient's disease state. A singleextra-cardiovascular lead 470 as shown in FIG. 12A may be replaced withthe transvenous three lead system as shown in FIG. 12B, and IMD 14 maybe programmed to change its operational configuration from thesingle-channel extra-cardiovascular configuration to the multi-channelconfiguration for use with the transvenous leads 410, 420 and 430.

Thus, a method and apparatus for delivering pacing pulses usingextra-cardiovascular electrodes have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or combination than the illustrative examples shown anddescribed herein. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

1-20. (canceled)
 21. A medical device comprising: a therapy modulehaving a capacitor array comprising a plurality of capacitors and anoutput signal line, the therapy module configured to generate acomposite pacing pulse comprising a series of at least two individualpulses by: generating a first pulse of the at least two individualpulses by selectively coupling a first portion of the plurality ofcapacitors to the output signal line, the selectively coupled firstportion of the plurality of capacitors having a first effectivecapacitance and the first pulse of the at least two individual pulseshaving a first decay rate corresponding to the first effectivecapacitance, and generating a second pulse of the at least twoindividual pulses by selectively coupling a second portion of theplurality of capacitors to the output signal line, the selectivelycoupled second portion of the plurality of capacitors having a secondeffective capacitance different than the first effective capacitance andthe second pulse of the at least two individual pulses having a seconddecay rate different that the first decay rate, the second decay ratecorresponding to the second effective capacitance.
 22. The device ofclaim 21, wherein the therapy module is further configured to: generatethe first pulse by coupling the first portion of the plurality ofcapacitors to the output signal line for a first pulse width; generatethe second pulse by coupling the second portion of the plurality ofcapacitors to the output signal line for a second pulse width, thesecond pulse width being different than the first pulse width.
 23. Thedevice of claim 22, wherein the therapy module is further configured to:set the first pulse width to terminate the first pulse decaying at thefirst decay rate at a first terminating voltage amplitude that is atleast a threshold voltage amplitude; and set the second pulse width toterminate the second pulse decaying at the second decay rate to a secondterminating voltage amplitude that is at least the threshold voltageamplitude.
 24. The device of claim 21, wherein the therapy module isconfigured to generate a third pulse of the composite pacing pulse byselectively coupling a third portion of the plurality of capacitors tothe output signal line, the third portion of the plurality of capacitorshaving a third effective capacitance.
 25. The device of claim 24,wherein the third effective capacitance is different than the secondeffective capacitance.
 26. The device of claim 24, wherein the therapymodule is configured to generate the third pulse by coupling the thirdportion of the plurality of capacitors to the output signal line for thefirst pulse width.
 27. The device of claim 24, wherein the therapymodule is configured to generate the composite pacing pulse by:generating the first pulse by coupling the first portion of theplurality of capacitors to the output signal line for the first pulsewidth; generating the third pulse of the composite pacing pulseconsecutively following the first pulse by coupling the third portion ofthe plurality of capacitors to the output signal line for the firstpulse width; and generating the second pulse consecutively following thethird pulse of the composite pacing pulse by coupling the second portionof the plurality of capacitors to the output signal line for the secondpulse width.
 28. The device of claim 27, wherein the therapy module isconfigured to: select the second portion of the plurality of capacitorsto have the second effective capacitance that is greater than the firsteffective capacitance and greater than the third effective capacitance;and set the second pulse width to be greater than the first pulse width.29. The device of claim 21, wherein the therapy module is configured to:generate the first pulse having a first pulse energy that is less than apacing capture threshold; generate the second pulse having a secondpulse energy that is less than the pacing capture threshold; andgenerate the composite pacing pulse comprising at least the first pulseand the second pulse, the composite pacing pulse having a cumulativepulse energy comprising at least the first pulse energy and the secondpulse energy that is at least the pacing capture threshold.
 30. Thedevice of claim 21, further comprising: switching circuitry configuredto sequentially couple the first portion of the plurality of capacitorsand the second portion of the plurality of capacitors to the outputline.
 31. A method comprising: generating a composite pacing pulsecomprising a series of at least two individual pulses by: generating afirst pulse of the at least two individual pulses by selectivelycoupling a first portion of a plurality of capacitors to an outputsignal line, the selectively coupled first portion of the plurality ofcapacitors having a first effective capacitance and the first pulse ofthe at least two individual pulses having a first decay ratecorresponding to the first effective capacitance, and generating asecond pulse of the at least two individual pulses by selectivelycoupling a second portion of the plurality of capacitors to the outputsignal line, the selectively coupled second portion of the plurality ofcapacitors having a second effective capacitance different than thefirst effective capacitance and the second pulse of the at least twoindividual pulses having a second decay rate different that the firstdecay rate, the second decay rate corresponding to the second effectivecapacitance.
 32. The method of claim 31, wherein: generating the firstpulse comprises coupling the first portion of the plurality ofcapacitors to the output signal line for a first pulse width; andgenerating the second pulse comprises coupling the second portion of theplurality of capacitors to the output signal line for a second pulsewidth, the second pulse width being different than the first pulsewidth.
 33. The method of claim 32, further comprising: setting the firstpulse width to terminate the first pulse decaying at the first decayrate at a first terminating voltage amplitude that is at least athreshold voltage amplitude; and setting the second pulse width toterminate the second pulse decaying at the second decay rate to a secondterminating voltage amplitude that is at least the threshold voltageamplitude.
 34. The method of claim 31, further comprising generating athird pulse of the composite pacing pulse by selectively coupling athird portion of the plurality of capacitors to the output signal line,the third portion of the plurality of capacitors having a thirdeffective capacitance.
 35. The method of claim 34, further comprisingselecting the third portion of the plurality of capacitors to have athird effective capacitance that is different than the second effectivecapacitance.
 36. The method of claim 34, further comprising generatingthe third pulse by coupling the third portion of the plurality ofcapacitors to the output signal line for the first pulse width.
 37. Themethod of claim 34, wherein generating the composite pacing pulsefurther comprises: generating the first pulse by coupling the firstportion of the plurality of capacitors to the output signal line for thefirst pulse width; generating the third pulse of the composite pacingpulse consecutively following the first pulse by coupling the thirdportion of the plurality of capacitors to the output signal line for thefirst pulse width; and generating the second pulse consecutivelyfollowing the third pulse of the composite pacing pulse by coupling thesecond portion of the plurality of capacitors to the output signal linefor the second pulse width.
 38. The method of claim 37, furthercomprising: selecting the second portion of the plurality of capacitorsto have the second effective capacitance that is greater than the firsteffective capacitance and greater than the third effective capacitance;and setting the second pulse width to be greater than the first pulsewidth.
 39. The method of claim 31, wherein generating the compositepacing pulse comprises: generating the first pulse having a first pulseenergy that is less than a pacing capture threshold; generating thesecond pulse having a second pulse energy that is less than the pacingcapture threshold; and generating the composite pacing pulse having acumulative pulse energy comprising at least the first pulse energy andthe second pulse energy that is at least the pacing capture threshold.40. The method of claim 31, further comprising: sequentially couplingthe first portion of the plurality of capacitors and the second portionof the plurality of capacitors to the output line via switchingcircuitry.
 41. A non-transitory, computer-readable storage mediumcomprising a set of instructions which, when executed by a controlmodule of a medical device, cause the medical device to: generate acomposite pacing pulse comprising a series of at least two individualpulses by: generating a first pulse of the at least two individualpulses by selectively coupling a first portion of a plurality ofcapacitors to an output signal line, the selectively coupled firstportion of the plurality of capacitors having a first effectivecapacitance and the first pulse of the at least two individual pulseshaving a first decay rate corresponding to the first effectivecapacitance, and generating a second pulse of the at least twoindividual pulses by selectively coupling a second portion of theplurality of capacitors to the output signal line, the selectivelycoupled second portion of the plurality of capacitors having a secondeffective capacitance different than the first effective capacitance andthe second pulse of the at least two individual pulses having a seconddecay rate different that the first decay rate, the second decay ratecorresponding to the second effective capacitance.